async-await: track nightly changes (#661)

The `tokio-async-await` crate is no longer a facade. Instead, the `tokio` crate
provides a feature flag to enable async/await support.

tokio-channel/src/async_await.rs

@@ -1,10 +0,0 @@
-use {oneshot, mpsc};
-
-use std::marker::Unpin;
-
-impl<T> Unpin for oneshot::Sender<T> {}
-impl<T> Unpin for oneshot::Receiver<T> {}
-
-impl<T> Unpin for mpsc::Sender<T> {}
-impl<T> Unpin for mpsc::UnboundedSender<T> {}
-impl<T> Unpin for mpsc::Receiver<T> {}

tokio-channel/src/lib.rs

@@ -1,8 +1,5 @@
 #![doc(html_root_url = "https://docs.rs/tokio-channel/0.1.0")]
 #![deny(missing_docs, warnings, missing_debug_implementations)]
-#![cfg_attr(feature = "async-await-preview", feature(
-        pin,
-        ))]
 
 //! Asynchronous channels.
 //!
@@ -15,8 +12,3 @@ pub mod mpsc;
 pub mod oneshot;
 
 mod lock;
-
-// ===== EXPERIMENTAL async / await support =====
-
-#[cfg(feature = "async-await-preview")]
-mod async_await;