Add capacity-related functions for VecMap

[aliblong]

Part of #18424

Adds functions reserve(uint), reserve_exact(uint), and capacity() to VecMap, as per the collections reform.

(Separation of PRs from #19516)

[Gankra]

I believe this is one place where our original plans as accepted in the RFC diverge from how we actually plan to do it. The new plans need to be accepted in a new RFC though. I am working on it with @aturon but we're both occupied for the next little-while.

While this patch seems to be more-or-less correct, I'd like to sit on it while we hash out conventions.

Sorry :(

[aliblong]

Okay, I'll close this for now, since I suspect the final version of capacity reservation for VecMap will look more like reserve_index than reserve.