sorted_array -- sorted one-dimensional immutable array

This takes an unsorted array and a compare function as an arguments and
returns a sorted one.

Non-mutating heap sort is used internally. This gives guaranteed performance in
O(n log n) comparisons and assignments for an array of size n.

This is a little wasteful on allocated memory, which is also O(n log n) since
partially sorted arrays are thrown away. This might be improved to use an
in-place heap sort to bring allocated memory down to O(n).