btree_multiset reference work in progress.
Rename headers consistent with unordered_map/set naming in the standard library. Remove obsolete dynamic_size header.
Remove index.hpp proof-of-concept header.
Change header and class names to conform to standard library unordered_set/map practice. index_set becomes btree_index to better reflect it being preferred over btree_index_map due to better safety when multiple indexes exist into the same flat file.
Add insert_unique_return_iterator_test.