Add List capacity and reserve(_) methods

[mhermier]

Exposing these List methods allow to grow the underlying container to a size reducing unnecessary allocations and copying, and can bring performance when the final size/required capacity of the list is known in advance.

[ruby0x1]

List.filled is related fwiw

[PureFox48]

A problem with List.filled is that sometimes you don't know how much capacity you'll need when creating the list.

This PR would enable us to increase the capacity subsequently in 'one hit' and, as it doesn't add much extra code, I think it would be worth doing.

[mhermier]

List.filled(_,_) as 2 side effect that this patch does not have. It does provides an internal buffer and List of the exact size. Even adding a removeAll() would be me costly to implement in user than what is provided here.

List.filled(_,_) could be extended to have a capacity argument, but I'm not sure if it is worth it.

[ruby0x1]

(related doesn't mean instead of, I'm aware of the value of the patch!)