feat(body): implement HttpBody for Request and Response

When the body type of a `Request` or `Response` implements `HttpBody`,
the `Request` or `Response` itself now implements `HttpBody`.

This allows writing things like `hyper::body::aggregate(req)` instead of
`hyper::body::aggregate(req.into_body())`.

Closes #2067

src/body/to_bytes.rs

@@ -5,7 +5,7 @@ use super::HttpBody;
 /// Concatenate the buffers from a body into a single `Bytes` asynchronously.
 ///
 /// This may require copying the data into a single buffer. If you don't need
-/// a contiguous buffer, prefer the [`aggregate`](crate::body::aggregate)
+/// a contiguous buffer, prefer the [`aggregate`](crate::body::aggregate())
 /// function.
 pub async fn to_bytes<T>(body: T) -> Result<Bytes, T::Error>
 where