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.

tokio-sync/src/task/mod.rs

@@ -0,0 +1,5 @@
+//! Thread-safe task notification primitives.
+
+mod atomic_task;
+
+pub use self::atomic_task::AtomicTask;