Counting Sort O(n) implementation based on CLRS
Call counting_sort(a, k), where a is the list to sort, k is the size of the language.
For example, if we are using a list of decimal numbers, k = 10. Since decimal numbers have 10 digits. If it was a list of characters, k = 26, et cetera.
Returns another sorted list. Runs in O(n) time, where n is the length of the list.