Streamlining the `WaitGroup` API about crystal

[stakach]

similar to my promise library too
https://github.com/spider-gazelle/promise?tab=readme-ov-file#promiseall

I'm all for this, much harder to shoot yourself in the foot and looks way better

[ysbaddaden]

RFC #3 had a #spawn(&) method but it looks it didn't make it to the pull request.

The .wait(&) method looks like a nice addition.

[straight-shoota]

WaitGroup is a low-level primitive and IMO shouldn't need to be super polished for convenience. The idea is that most people wouldn't use it directly, in favour of a higher level concurrency API (cf. #6468). At the moment such a higher level API is still missing. But once we complete that, I suppose there would be little reason to use WaitGroup directly.

[bararchy]

I think spawn makes sense even for a low level API?
btw, would WaitGroup work for something like

select
when bail.receive
when wg.wait
end

That would allow to make it a basis in a worker-pool implementation, or maybe even have some wg.release mechanism for when you want multiple workers to do something but first one done means we can release the others.