std: Rename unstable::mutex::Mutex to StaticNativeMutex.

This better reflects its purpose and design.

src/libstd/os.rs

@@ -144,9 +144,9 @@ Accessing environment variables is not generally threadsafe.
 Serialize access through a global lock.
 */
 fn with_env_lock<T>(f: || -> T) -> T {
-    use unstable::mutex::{Mutex, MUTEX_INIT};
+    use unstable::mutex::{StaticNativeMutex, NATIVE_MUTEX_INIT};
 
-    static mut lock: Mutex = MUTEX_INIT;
+    static mut lock: StaticNativeMutex = NATIVE_MUTEX_INIT;
 
     unsafe {
         let _guard = lock.lock();