Renames UnfairLock to TSMutex and refreshes its interface to remove methods
that are unused except in tests.
TSMutex has a generic parameter State that supports non-sendable and
non-copyable values. UnfairLock is a type alias of TSMutex<Void>.
The withLock() function is also no longer inherited from NSLocking, instead
being implemented to be similar to OSAllocatedUnfairLock's API.
Finally, the class is audited for sendability and compiles without warnings
under strict concurrency checking.
