feat(body): add body::aggregate and body::to_bytes functions

Adds utility functions to `hyper::body` to help asynchronously
collecting all the buffers of some `HttpBody` into one.

- `aggregate` will collect all into an `impl Buf` without copying the
  contents. This is ideal if you don't need a contiguous buffer.
- `to_bytes` will copy all the data into a single contiguous `Bytes`
  buffer.

examples/client.rs

@@ -40,6 +40,8 @@ async fn fetch_url(url: hyper::Uri) -> Result<()> {
     println!("Response: {}", res.status());
     println!("Headers: {:#?}\n", res.headers());
 
+    // 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 {
         let chunk = next?;
         io::stdout().write_all(&chunk).await?;