Yet another WaitGroup implementation.
None of the existing crates fit my needs exactly, so here's one more that (hopefully) will.
Highlights:
- Generalizes "tasks" to [Ref]s. More of a change in nomenclature than anything else. It's not always a group of tasks you're waiting on - it could be that you're waiting on a gaggle of structs to all be dropped.
- [Ref]s and [Waiter]s are entirely disjoint. You don't need a [Waiter] to create a new [Ref].
- Everything is cloneable and behaves as one would expect - cloned [Ref]s will all block every cloned [Waiter], which can be awaited concurrently.