Fix clippy::mem_replace_with_default warning

```
warning: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
  --> futures-util/src/future/select_all.rs:62:28
   |
62 |                 let rest = mem::replace(&mut self.inner, Vec::new());
   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(&mut self.inner)`
   |
   = note: `#[warn(clippy::mem_replace_with_default)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default

warning: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
  --> futures-util/src/future/select_ok.rs:62:40
   |
62 | ...                   let rest = mem::replace(&mut self.inner, Vec::new());
   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(&mut self.inner)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default

warning: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
  --> futures-util/src/stream/stream/collect.rs:22:9
   |
22 |         mem::replace(self.project().collection, Default::default())
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(self.project().collection)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default

warning: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
  --> futures-util/src/stream/stream/unzip.rs:24:10
   |
24 |         (mem::replace(this.left, Default::default()), mem::replace(this.right, Default::default()))
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(this.left)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default

warning: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
  --> futures-util/src/stream/stream/unzip.rs:24:55
   |
24 |         (mem::replace(this.left, Default::default()), mem::replace(this.right, Default::default()))
   |                                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(this.right)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default

warning: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
  --> futures-util/src/stream/stream/chunks.rs:69:40
   |
69 |                         let full_buf = mem::replace(this.items, Vec::new());
   |                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(this.items)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default

warning: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
  --> futures-util/src/stream/stream/ready_chunks.rs:77:40
   |
77 |                         let full_buf = mem::replace(this.items, Vec::new());
   |                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(this.items)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default

warning: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
  --> futures-util/src/stream/try_stream/try_collect.rs:48:31
   |
48 |                 None => break mem::replace(this.items, Default::default()),
   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(this.items)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default

warning: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
  --> futures-util/src/stream/try_stream/try_chunks.rs:74:40
   |
74 |                         let full_buf = mem::replace(this.items, Vec::new());
   |                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(this.items)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default

warning: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
  --> futures-util/src/io/lines.rs:45:29
   |
45 |         Poll::Ready(Some(Ok(mem::replace(this.buf, String::new()))))
   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(this.buf)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default

warning: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
  --> futures-util/src/io/read_exact.rs:33:33
   |
33 |                 let (_, rest) = mem::replace(&mut this.buf, &mut []).split_at_mut(n);
   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(&mut this.buf)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default

warning: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
  --> futures-util/src/io/read_line.rs:25:31
   |
25 |         Self { reader, bytes: mem::replace(buf, String::new()).into_bytes(), buf, read: 0 }
   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(buf)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default

warning: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
  --> futures-util/src/io/read_to_string.rs:25:31
   |
25 |         Self { reader, bytes: mem::replace(buf, String::new()).into_bytes(), buf, start_len }
   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(buf)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default

warning: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
  --> futures-util/src/io/write_all.rs:33:33
   |
33 |                 let (_, rest) = mem::replace(&mut this.buf, &[]).split_at(n);
   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(&mut this.buf)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default

warning: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
   --> futures/tests/sink.rs:141:9
    |
141 |         mem::replace(&mut self.data, Vec::new())
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(&mut self.data)`
    |
    = note: `#[warn(clippy::mem_replace_with_default)]` on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default
```

Author: taiki-e

futures-util/src/future/select_all.rs

@@ -59,7 +59,7 @@ impl<Fut: Future + Unpin> Future for SelectAll<Fut> {
         match item {
             Some((idx, res)) => {
                 let _ = self.inner.swap_remove(idx);
-                let rest = mem::replace(&mut self.inner, Vec::new());
+                let rest = mem::take(&mut self.inner);
                 Poll::Ready((res, idx, rest))
             }
             None => Poll::Pending,

futures-util/src/future/select_ok.rs

@@ -59,7 +59,7 @@ impl<Fut: TryFuture + Unpin> Future for SelectOk<Fut> {
                     drop(self.inner.remove(idx));
                     match res {
                         Ok(e) => {
-                            let rest = mem::replace(&mut self.inner, Vec::new());
+                            let rest = mem::take(&mut self.inner);
                             return Poll::Ready(Ok((e, rest)));
                         }
                         Err(e) => {

futures-util/src/io/lines.rs

@@ -42,6 +42,6 @@ impl<R: AsyncBufRead> Stream for Lines<R> {
                 this.buf.pop();
             }
         }
-        Poll::Ready(Some(Ok(mem::replace(this.buf, String::new()))))
+        Poll::Ready(Some(Ok(mem::take(this.buf))))
     }
 }

futures-util/src/io/read_exact.rs

@@ -30,7 +30,7 @@ impl<R: AsyncRead + ?Sized + Unpin> Future for ReadExact<'_, R> {
         while !this.buf.is_empty() {
             let n = ready!(Pin::new(&mut this.reader).poll_read(cx, this.buf))?;
             {
-                let (_, rest) = mem::replace(&mut this.buf, &mut []).split_at_mut(n);
+                let (_, rest) = mem::take(&mut this.buf).split_at_mut(n);
                 this.buf = rest;
             }
             if n == 0 {

futures-util/src/io/read_line.rs

@@ -22,7 +22,7 @@ impl<R: ?Sized + Unpin> Unpin for ReadLine<'_, R> {}
 
 impl<'a, R: AsyncBufRead + ?Sized + Unpin> ReadLine<'a, R> {
     pub(super) fn new(reader: &'a mut R, buf: &'a mut String) -> Self {
-        Self { reader, bytes: mem::replace(buf, String::new()).into_bytes(), buf, read: 0 }
+        Self { reader, bytes: mem::take(buf).into_bytes(), buf, read: 0 }
     }
 }
 

futures-util/src/io/read_to_string.rs

@@ -22,7 +22,7 @@ impl<R: ?Sized + Unpin> Unpin for ReadToString<'_, R> {}
 impl<'a, R: AsyncRead + ?Sized + Unpin> ReadToString<'a, R> {
     pub(super) fn new(reader: &'a mut R, buf: &'a mut String) -> Self {
         let start_len = buf.len();
-        Self { reader, bytes: mem::replace(buf, String::new()).into_bytes(), buf, start_len }
+        Self { reader, bytes: mem::take(buf).into_bytes(), buf, start_len }
     }
 }
 

futures-util/src/io/write_all.rs

@@ -30,7 +30,7 @@ impl<W: AsyncWrite + ?Sized + Unpin> Future for WriteAll<'_, W> {
         while !this.buf.is_empty() {
             let n = ready!(Pin::new(&mut this.writer).poll_write(cx, this.buf))?;
             {
-                let (_, rest) = mem::replace(&mut this.buf, &[]).split_at(n);
+                let (_, rest) = mem::take(&mut this.buf).split_at(n);
                 this.buf = rest;
             }
             if n == 0 {

futures-util/src/stream/stream/chunks.rs

@@ -66,7 +66,7 @@ impl<St: Stream> Stream for Chunks<St> {
                     let last = if this.items.is_empty() {
                         None
                     } else {
-                        let full_buf = mem::replace(this.items, Vec::new());
+                        let full_buf = mem::take(this.items);
                         Some(full_buf)
                     };
 

futures-util/src/stream/stream/collect.rs

@@ -19,7 +19,7 @@ pin_project! {
 
 impl<St: Stream, C: Default> Collect<St, C> {
     fn finish(self: Pin<&mut Self>) -> C {
-        mem::replace(self.project().collection, Default::default())
+        mem::take(self.project().collection)
     }
 
     pub(super) fn new(stream: St) -> Self {

futures-util/src/stream/stream/ready_chunks.rs

@@ -74,7 +74,7 @@ impl<St: Stream> Stream for ReadyChunks<St> {
                     let last = if this.items.is_empty() {
                         None
                     } else {
-                        let full_buf = mem::replace(this.items, Vec::new());
+                        let full_buf = mem::take(this.items);
                         Some(full_buf)
                     };