Linux futex-based implementations of Mutex, RwLock, Condvar, Once, and OnceLock, as well as RawMutex, RawRwLock, and RawCondvar, derived from the futex code in std, factored out to a standalone no_std crate using rustix to do the futex and lock_api to provide most of the public Mutex and RwLock API.

lock_api does not support poisoning, so support for poisoning is omitted.

In this library, Condvar, RawCondvar, RawMutex, and Once are guaranteed to be repr(transparent) wrappers around a single AtomicU32. RawRwLock is guaranteed to be a repr(C) wrapper around two AtomicU32s. The contents of these AtomicU32s are not documented, except that all these types' const fn new() and INIT are guaranteed to initialize them to all zeros.