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

This better reflects its purpose and design.

src/libstd/unstable/dynamic_lib.rs

@@ -152,8 +152,8 @@ pub mod dl {
     }
 
     pub fn check_for_errors_in<T>(f: || -> T) -> Result<T, ~str> {
-        use unstable::mutex::{Mutex, MUTEX_INIT};
-        static mut lock: Mutex = MUTEX_INIT;
+        use unstable::mutex::{StaticNativeMutex, NATIVE_MUTEX_INIT};
+        static mut lock: StaticNativeMutex = NATIVE_MUTEX_INIT;
         unsafe {
             // dlerror isn't thread safe, so we need to lock around this entire
             // sequence