review: use &mut self method receivers

this applies a review suggestion here:
https://github.com/hyperium/http-body/pull/100/files#r1399780355

this commit refactors the channel-backed body in hyperium#100, changing the
signature of `send_*` methods on the sender to require a mutable
reference.

http-body-util/src/channel.rs

@@ -79,17 +79,17 @@ pub struct Sender<D, E = std::convert::Infallible> {
 
 impl<D, E> Sender<D, E> {
     /// Send a frame on the channel.
-    pub async fn send(&self, frame: Frame<D>) -> Result<(), SendError> {
+    pub async fn send(&mut self, frame: Frame<D>) -> Result<(), SendError> {
         self.tx_frame.send(frame).await.map_err(|_| SendError)
     }
 
     /// Send data on data channel.
-    pub async fn send_data(&self, buf: D) -> Result<(), SendError> {
+    pub async fn send_data(&mut self, buf: D) -> Result<(), SendError> {
         self.send(Frame::data(buf)).await
     }
 
     /// Send trailers on trailers channel.
-    pub async fn send_trailers(&self, trailers: HeaderMap) -> Result<(), SendError> {
+    pub async fn send_trailers(&mut self, trailers: HeaderMap) -> Result<(), SendError> {
         self.send(Frame::trailers(trailers)).await
     }