Implement atomic-await

[krtab]

Hi!

Thanks a lot for this very impressive crate.

I'm trying to use it on rendezvous but I can't because I'm depending on @m-ou-se atomic-wait.

I've tried using mock implementations like:

#[cfg(loom)]
mod atomic_wait {
    pub use loom::sync::atomic::AtomicU32;
    pub fn wake_all(_atomic: *const AtomicU32) {
        loom::thread::yield_now();
    }
    pub fn wait(_atomic: &AtomicU32, _value: u32) {
        loom::thread::yield_now();
    }

}

but this unfortunately leads to "Model exceeded maximum number of branches. This is often caused by an algorithm requiring the processor to make progress, e.g. spin locks." in some cases that would be relevant to the tests.

If feasible, I think that having atomic-wait supported by loom would be great!

Thanks