Array == Difference
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.
arrayDiff([1,2],) == 
If a value is present in b, all of its occurrences must be removed from the other:
arrayDiff([1,2,2,2,3],) == [1,3]