Add unordered array element remove

[timothyqiu]

remove_at(index) shifts every element after index. This has a significant performance cost if the array contains many elements.

For order-insensitive usages (e.g. using the array as a set), removing an element can be implemented as copying the last element to index. This is cheaper.

So I suggest adding something like remove_at_unordered(index) to array types.