Your goal in this kata is to implement a difference function, which subtracts
one list from another and returns the result.

It should remove all values from list a, which are present in list b keeping
their order.

array_diff(vec![1,2], vec![1]) == vec![2] If a value is present in b, all of its
occurrences must be removed from the other:

array_diff(vec![1,2,2,2,3], vec![2]) == vec![1,3]