io: change AsyncRead to use a ReadBuf (#2758)

Works towards #2716. Changes the argument to `AsyncRead::poll_read` to
take a `ReadBuf` struct that safely manages writes to uninitialized memory.

tokio-util/tests/framed.rs

@@ -55,8 +55,8 @@ impl AsyncRead for DontReadIntoThis {
     fn poll_read(
         self: Pin<&mut Self>,
         _cx: &mut Context<'_>,
-        _buf: &mut [u8],
-    ) -> Poll<io::Result<usize>> {
+        _buf: &mut tokio::io::ReadBuf<'_>,
+    ) -> Poll<io::Result<()>> {
         unreachable!()
     }
 }