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

examples/client.rs

@@ -42,7 +42,7 @@ async fn fetch_url(url: hyper::Uri) -> Result<()> {
 
     // Stream the body, writing each chunk to stdout as we get it
     // (instead of buffering and printing at the end).
-    while let Some(next) = res.body_mut().data().await {
+    while let Some(next) = res.data().await {
         let chunk = next?;
         io::stdout().write_all(&chunk).await?;
     }