IntList is a class for working with lists of primitive ints without
boxing them into Integers. For writing the reverse index file format,
sorting ints will be needed but IntList doesn't provide a sorting
method yet.
Add the #sort method to sort an IntList by an IntComparator, using
quicksort, which has a average runtime of O(n log n) and sorts in-place
by using O(log n) stack frames for recursive calls.
Change-Id: Id69a687c8a16d46b13b28783b194a880f3f4c437
Signed-off-by: Anna Papitto <annapapitto@google.com>