Merge libsync into libstd

This patch merges the `libsync` crate into `libstd`, undoing part of the
facade. This is in preparation for ultimately merging `librustrt`, as
well as the upcoming rewrite of `sync`.

Because this removes the `libsync` crate, it is a:

[breaking-change]

However, all uses of `libsync` should be able to reroute through
`std::sync` and `std::comm` instead.

src/libstd/lib.rs

@@ -124,7 +124,6 @@ extern crate unicode;
 extern crate core;
 extern crate "collections" as core_collections;
 extern crate "rand" as core_rand;
-extern crate "sync" as core_sync;
 extern crate libc;
 extern crate rustrt;
 
@@ -173,8 +172,6 @@ pub use rustrt::c_str;
 
 pub use unicode::char;
 
-pub use core_sync::comm;
-
 /* Exported macros */
 
 pub mod macros;
@@ -236,6 +233,7 @@ pub mod hash;
 
 pub mod task;
 pub mod sync;
+pub mod comm;
 
 #[cfg(unix)]
 #[path = "sys/unix/mod.rs"] mod sys;