Array reallocation API

[jkthorne]

I think there should be an API for resizing an array. This is useful when you know you are about to insert a large amount of data and you only want one allocation of memory and one memory copy.

This is a followup from a conversation on the array buffer refactor. https://github.com/crystal-lang/crystal/pull/8166#issuecomment-534233809

Pros: Gives more flexibility in memory allocations and the potential for performance.

Cons: More APIs to support and is a more advanced feature.

[rdp]

So you don't know the size when you first create it, I presume? Maybe make resize_to_capacity public?

[beta-ziliani]

Note: This is already addressed in #11485 (Array#ensure_capacity).