Implement arrays utility library instead of our own

https://github.com/phpfunct/funct seems to cover it all for both strings and arrays but isn't actively maintained.

https://github.com/voku/arrayy is from a trusted source but doesn't seem to add the value we're aiming for.

Arrays:
* contains
* diff
* intersect
* flatten
* get first
* get last
* recursive count of items