refactor: add cmp function on arraylists

[callumgran]

In adding a comparison function that returns a signed integer, you can write more efficient and more complex functions for testing equality and even sorting of elements in the arraylist.

This would allow f.eks for arraylist_sort() that is a wrapper for qsort or another sort function that would then just use the cmp function from the arraylist.

This also makes it easier to find the correct item in the arraylist if thou so wish.