Resolve mem_replace_with_default clippy lint

    warning: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
       --> src/expand.rs:195:30
        |
    195 |                 let bounds = mem::replace(&mut param.bounds, Punctuated::new());
        |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(&mut param.bounds)`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default
        = note: `-W clippy::mem-replace-with-default` implied by `-W clippy::all`
        = help: to override `-W clippy::all` add `#[allow(clippy::mem_replace_with_default)]`

    warning: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
       --> src/expand.rs:206:30
        |
    206 |                 let bounds = mem::replace(&mut param.bounds, Punctuated::new());
        |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(&mut param.bounds)`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default

Author: dtolnay

src/expand.rs

@@ -192,7 +192,7 @@ fn transform_sig(
                     Some(colon_token) => colon_token.span,
                     None => param_name.span(),
                 };
-                let bounds = mem::replace(&mut param.bounds, Punctuated::new());
+                let bounds = mem::take(&mut param.bounds);
                 where_clause_or_default(&mut sig.generics.where_clause)
                     .predicates
                     .push(parse_quote_spanned!(span=> #param_name: 'async_trait + #bounds));
@@ -203,7 +203,7 @@ fn transform_sig(
                     Some(colon_token) => colon_token.span,
                     None => param_name.span(),
                 };
-                let bounds = mem::replace(&mut param.bounds, Punctuated::new());
+                let bounds = mem::take(&mut param.bounds);
                 where_clause_or_default(&mut sig.generics.where_clause)
                     .predicates
                     .push(parse_quote_spanned!(span=> #param: 'async_trait + #bounds));