RedBlackTreeSet could have a pop_{first, last} methods

[Eh2406]

I was experimenting with using a RedBlackTree as a priority queue and ended up with:

let next_work = queue.first();
if let Some(k) = next_work {
    queue.remove_mut(k);
}

It would be nice to have a pop_first that removed the first item and returned it in one.

[orium]

Sounds good. If you want to take it and do a PR go for it :)

[Eh2406]

looks like std calls somthing similar take.

As to doing a PR, I do not know the internals of rpds. I don't think I will have the spare cycles to figure it out.