- Move `held` into the boxed part, since the SRW lock implementation does not use this. This makes the Mutex 50% smaller. - Use `Cell` instead of `UnsafeCell` for `held`, such that `.replace()` can be used. - Add some comments.
- Move `held` into the boxed part, since the SRW lock implementation does not use this. This makes the Mutex 50% smaller. - Use `Cell` instead of `UnsafeCell` for `held`, such that `.replace()` can be used. - Add some comments.