Make it possible to have unboxed mutexes on specific platforms.

This commit keeps all mutexes boxed on all platforms, but makes it
trivial to remove the box on some platforms later.

library/std/src/sys/unix/mutex.rs

@@ -5,6 +5,8 @@ pub struct Mutex {
     inner: UnsafeCell<libc::pthread_mutex_t>,
 }
 
+pub type MovableMutex = Box<Mutex>;
+
 #[inline]
 pub unsafe fn raw(m: &Mutex) -> *mut libc::pthread_mutex_t {
     m.inner.get()