Rollup merge of rust-lang#94818 - yoshuawuyts:into-future-associated-type, r=joshtriplett

Rename `IntoFuture::Future` to `IntoFuture::IntoFuture`

Ref: rust-lang#67644 (comment)

This renames `IntoFuture::Future` to `IntoFuture::IntoFuture`. This adds the `Into*` prefix to the associated type, similar to the [`IntoIterator::IntoIter`](https://doc.rust-lang.org/std/iter/trait.IntoIterator.html#associatedtype.IntoIter) associated type. It's my mistake we didn't do so in the first place. This fixes that and brings the two closer together. Thanks!

### References
__`IntoIterator` trait def__
```rust
pub trait IntoIterator {
    type Item;
    type IntoIter: Iterator<Item = Self::Item>;
    fn into_iter(self) -> Self::IntoIter;
}
```
__`IntoFuture` trait def__
```rust
pub trait IntoFuture {
    type Output;
    type IntoFuture: Future<Output = Self::Output>; // Prior to this PR: `type Future:`
    fn into_future(self) -> Self::IntoFuture;
}
```

cc/ `@eholk` `@rust-lang/wg-async`

Author: Dylan-DPC

core/src/future/into_future.rs

@@ -9,20 +9,20 @@ pub trait IntoFuture {
 
     /// Which kind of future are we turning this into?
     #[unstable(feature = "into_future", issue = "67644")]
-    type Future: Future<Output = Self::Output>;
+    type IntoFuture: Future<Output = Self::Output>;
 
     /// Creates a future from a value.
     #[unstable(feature = "into_future", issue = "67644")]
     #[lang = "into_future"]
-    fn into_future(self) -> Self::Future;
+    fn into_future(self) -> Self::IntoFuture;
 }
 
 #[unstable(feature = "into_future", issue = "67644")]
 impl<F: Future> IntoFuture for F {
     type Output = F::Output;
-    type Future = F;
+    type IntoFuture = F;
 
-    fn into_future(self) -> Self::Future {
+    fn into_future(self) -> Self::IntoFuture {
         self
     }
 }