Auto merge of #76645 - fusion-engineering-forks:windows-lock, r=kennytm

Small cleanups in Windows Mutex.

 - 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.
 - Avoid creating multiple `&mut`s to the critical section object in `ReentrantMutex`.

library/std/src/lib.rs

@@ -315,6 +315,7 @@
 #![feature(try_reserve)]
 #![feature(unboxed_closures)]
 #![feature(unsafe_block_in_unsafe_fn)]
+#![feature(unsafe_cell_raw_get)]
 #![feature(untagged_unions)]
 #![feature(unwind_attributes)]
 #![feature(vec_into_raw_parts)]