Implement arrays utility library instead of our own

[cliffordp]

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