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.