Implement basic std functions to manipulate arrays

[mks-h]

Implements some standard library functions that allow to manipulate arrays:

1. last(array) to get the last element (merely a shorthand for array[len(array) - 1])
2. remove_at(array, index) to remove an element at the index, translates to unset array[index]
3. extract_at(array, index) — same as remove_at, but also returns the element
4. pop(array) — remove and return the last element
5. shift(array) — remove and return the first element

[Mte90]

So checking with the function we have right now they are very useful :-)

[Mte90]

This was reported on discord about this PR: The issue right now is that the length of an array doesn't update after remove_at operation this means that we need to test this code and maybe do some fixes.

[Mte90]

@hdwalters some tests failed

[hdwalters]

@hdwalters some tests failed

Yes I know. It's because standard library function remove_at is failing to update the passed-by-ref array:

pub fun remove_at(ref array: [], index: Num): Null {
    trust $ unset {nameof array}[{index}] $
}

This is a flawed approach, because unset in Bash doesn't actually shift the following items up to fill the gap, it just marks the item at that index as missing (so removing at the same index is a no-op). I'm working on a better solution.

[hdwalters]

I created a new pull request #625, as my changes did not show up when I pushed to this branch.

[hdwalters]

I created a third pull request #626, sourcing a branch on my fork of the repo.