merge_arys takes two sorted arys and merge them into one sorted array
Implement Mergesort algorithm

* Add two tests
* Return only element if array.size is 1 (base case)
* Divide and conquer! Divide array into left and right and apply mergesort to each side
* Force parameters to be arrays by putting the return of mergesort into
	an array and flattening it
Reduce code

* Return full array (with one element) instead of first element so
	there's no need for creating an array and flattenig it