ArraySort provides a stable implementation of merge sort through its
method. It should be used instead of
Array.sort in cases where the order
of equal elements has to be retained on all targets.
a according to the comparison function
cmp(x,y) returns 0 if
x == y, a positive Int if
x > y and a
negative Int if
x < y.
This operation modifies Array
a in place.
This operation is stable: The order of equal elements is preserved.
cmp are null, the result is unspecified.