Async-std's waker_set.rs

This crate is just the waker set from async_std::sync::waker_set, which I couldn't find in a standalone crate.

If upstream updates, please file an issue or PR here. All other issues and PRs should go to async-std.

See also:

• waitlist, with different performance tradeoffs

lib.rs:

A common utility for building synchronization primitives.

When an async operation is blocked, it needs to register itself somewhere so that it can be notified later on. The WakerSet type helps with keeping track of such async operations and notifying them when they may make progress.