Introduce tokio-sync crate containing synchronization primitives. (#839)

Introduce a tokio-sync crate containing useful synchronization primitives for programs
written using Tokio.

The initial release contains:

* An mpsc channel
* A oneshot channel
* A semaphore implementation
* An `AtomicTask` primitive.

The `oneshot` and `mpsc` channels are new implementations providing improved
performance characteristics. In some benchmarks, the new mpsc channel shows
up to 7x improvement over the version provided by the `futures` crate. Unfortunately,
the `oneshot` implementation only provides a slight performance improvement as it
is mostly limited by the `futures` 0.1 task system. Once updated to the `std` version
of `Future` (currently nightly only), much greater performance improvements should
be achievable by `oneshot`.

Additionally, he implementations provided here are checked using
[Loom](http://github.com/carllerche/loom/), which provides greater confidence of
correctness.

src/lib.rs

@@ -100,6 +100,8 @@ extern crate tokio_fs;
 extern crate tokio_reactor;
 #[cfg(feature = "rt-full")]
 extern crate tokio_threadpool;
+#[cfg(feature = "sync")]
+extern crate tokio_sync;
 #[cfg(feature = "timer")]
 extern crate tokio_timer;
 #[cfg(feature = "tcp")]
@@ -126,6 +128,8 @@ pub mod net;
 pub mod prelude;
 #[cfg(feature = "reactor")]
 pub mod reactor;
+#[cfg(feature = "sync")]
+pub mod sync;
 #[cfg(feature = "timer")]
 pub mod timer;
 pub mod util;

src/sync.rs

@@ -0,0 +1,16 @@
+//! Future-aware synchronization
+//!
+//! This module is enabled with the **`sync`** feature flag.
+//!
+//! Tasks sometimes need to communicate with each other. This module contains
+//! two basic abstractions for doing so:
+//!
+//! - [oneshot](oneshot/index.html), a way of sending a single value
+//!   from one task to another.
+//! - [mpsc](mpsc/index.html), a multi-producer, single-consumer channel for
+//!   sending values between tasks.
+
+pub use tokio_sync::{
+    mpsc,
+    oneshot,
+};