Implement basic std functions to manipulate arrays

[hdwalters]

Implements some standard library functions that allow to manipulate arrays:

1. first(array) to get the first element (merely a shorthand for array[0])
2. last(array) to get the last element (merely a shorthand for array[len(array) - 1])
3. remove_at(array, index) to remove an element at the index, translates to unset array[index]
4. extract_at(array, index) — same as remove_at, but also returns the element
5. pop(array) — remove and return the last element
6. shift(array) — remove and return the first element

[hdwalters]

Closed this PR in favour of #626, which has a branch (with a more descriptive source name) hosted in my own fork of the repo.