Add wait_for and wait_for_value to WaitCell and WaitQueue

[jamesmunns]

Implementing the interface that @hawkw so wisely suggested

[hawkw]

This looks great, thank you! I left some feedback, but it's mostly just me being pedantic about docs, plus some style suggestions that you're welcome to take if you like them, or to ignore if you don't.

It might be nice to have tests and/or examples for the new code, but it's simple enough that I'm happy to merge this regardless. We may want to add tests asserting that the futures returned by these functions are Send/Sync when the input closure is Send/Sync, though, so that future internal changes don't introduce an accidental API compatibility breakage.

[jamesmunns]

github was being weird about applying suggestions, not sure why it thought they were overlapping. I plan to come back to add docs and address the remaining open topics

[hawkw]

Great, I'll be happy to merge this whenever the docs are ready (and ideally, when we can get a clean CI build on main 😬)

[hawkw]

docs and tests look good to me --- happy to merge this once CI is green!

[jamesmunns]

@hawkw tests are green (🎉), let me know what you think re: std and subscribe.

[hawkw]

merged, thank you!