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`.

examples/simple.rs

@@ -2,7 +2,7 @@
 
 #[tokio::main]
 async fn main() -> Result<(), reqwest::Error> {
-    let mut res = reqwest::Client::new()
+    let res = reqwest::Client::new()
         .get("https://hyper.rs")
         .send()
         .await?;