add experimental async/await support. (#582)

This patch adds experimental async/await support to Tokio. It does this
by adding feature flags to existing libs only where necessary in order
to add nightly specific code (mostly `Unpin` implementations). It then
provides a new crate: `tokio-async-await` which is a shim layer on top
of `tokio`.

The `tokio-async-await` crate is expected to look exactly like `tokio`
does, but with async / await support. This strategy reduces the amount
of cfg guarding in the main libraries.

This patch also adds `tokio-channel`, which is copied from futures-rs
0.1 and adds the necessary `Unpin` implementations. In general, futures
0.1 is mostly unmaintained, so it will make sense for Tokio to take over
maintainership of key components regardless of async / await support.

tokio-async-await/examples/echo_server.rs

@@ -0,0 +1,45 @@
+#![feature(await_macro, async_await)]
+
+#[macro_use]
+extern crate tokio;
+
+use tokio::net::{TcpListener, TcpStream};
+use tokio::prelude::*;
+
+use std::net::SocketAddr;
+
+fn handle(mut stream: TcpStream) {
+    tokio::spawn_async(async move {
+        let mut buf = [0; 1024];
+
+        loop {
+            match await!(stream.read_async(&mut buf)).unwrap() {
+                0 => break, // Socket closed
+                n => {
+                    // Send the data back
+                    await!(stream.write_all_async(&buf[0..n])).unwrap();
+                }
+            }
+        }
+    });
+}
+
+fn main() {
+    use std::env;
+
+    let addr = env::args().nth(1).unwrap_or("127.0.0.1:8080".to_string());
+    let addr = addr.parse::<SocketAddr>().unwrap();
+
+    // Bind the TCP listener
+    let listener = TcpListener::bind(&addr).unwrap();
+    println!("Listening on: {}", addr);
+
+    tokio::run_async(async {
+        let mut incoming = listener.incoming();
+
+        while let Some(stream) = await!(incoming.next()) {
+            let stream = stream.unwrap();
+            handle(stream);
+        }
+    });
+}