std: Introduce std::sync

For now, this moves the following modules to std::sync

* UnsafeArc (also removed unwrap method)
* mpsc_queue
* spsc_queue
* atomics
* mpmc_bounded_queue
* deque

We may want to remove some of the queues, but for now this moves things out of
std::rt into std::sync

src/libstd/lib.rs

@@ -159,6 +159,7 @@ pub mod trie;
 pub mod task;
 pub mod comm;
 pub mod local_data;
+pub mod sync;
 
 
 /* Runtime and platform support */