ps_set -- a partially sorted set based on ps_map

ps_set is a persistent set of ordered values. This set is generally
well-behaved with respect to cumulative and average performance.

WARNING: Due to the high worst-case time for addition, this structure should
not be used in situations when adding a single element repeatedly to the same
instance of ps_set is performance critical. If the resulting set's size n is a
power of 2, this will trigger the worst-case addition time resulting in
O(m*n log² n) for adding an element m times.