Rewrite sync::mutex as thin layer over native mutexes

Previously, sync::mutex had to split between green and native runtime
systems and thus could not simply use the native mutex facility.

This commit rewrites sync::mutex to link directly to native mutexes; in
the future, the two will probably be coalesced into a single
module (once librustrt is pulled into libstd wholesale).

Author: aturon

src/libsync/lib.rs

@@ -54,7 +54,6 @@ pub mod atomic;
 
 // Concurrent data structures
 
-mod mpsc_intrusive;
 pub mod spsc_queue;
 pub mod mpsc_queue;
 pub mod mpmc_bounded_queue;