rank_filter.hxx: Replaced std::multiset with a typdef to make it easier to swap out implementations.
rank_filter.hxx: Replaced std::multiset with a typdef to make it easier to swap out implementations.

jakirkham/rank_filter,jakirkham/rank_filter,nanshe-org/rank_filter,nanshe-org/rank_filter,DudLab/rank_filter,nanshe-org/rank_filter,jakirkham/rank_filter,DudLab/rank_filter,DudLab/rank_filter