[feat] Async Notify

[JonasKruckenberg]

Implement/port an asynchronous Notify type that allows any number of tasks to wait on a signal and other tasks to wake them up by notifying one or many at once. See tokio's Notify and async_std's Condvar for inspiration.

This is not only a useful general purpose sync primitive, it is also a requirement for the WASM memory.atomic.wait and memory.atomic.notify instructions and potentially any page-eviction, page-compaction deamon tasks in the future.