[algorithm.partition] (bug-fix:minor) When a partition is created, there is no previous partition.
[algorithm] (change:major) Finalization of subset implementation and test cases.
[algorithm] Added subset header
[algorithm] (change:minor) Minor improvement.
[algorithm] (new:minor) Added functions for next/previous partial permutations.
[algorithm] (change:minor) Renamed iterator types in subset_traits class.