refine async API

- Converted `Response::text` and `Response::json` to `async fn`
- Added `Response::bytes` async fn as a counterpat to `text`.
- Added `Response::chunk` async fn to stream chunks of the response body.
- Added `From<Response> for Body` to allow piping a response as a request body.
- Removed `Decoder` from public API
- Removed body accessor methods from `Response`
- Removed `Chunk` type, replaced with `bytes::Bytes`.
- Removed public `impl Stream for Body`.

tests/timeouts.rs

@@ -142,7 +142,7 @@ fn test_read_timeout() {
     };
 
     let url = format!("http://{}/read-timeout", server.addr());
-    let mut res = reqwest::blocking::Client::builder()
+    let res = reqwest::blocking::Client::builder()
         .timeout(Duration::from_millis(500))
         .build()
         .unwrap()