Add initial flat file adapters to helpers.hpp
Separate helpers.hpp into two headers; index_helpers.hpp being specific to indexes, and helpers.hpp being pure btree helpers. Better separation of concerns and limits btree header inclusion to its smaller set of dependencies.
Describe possible design strategy for dealing with maps having keys living in the btree vs maps having keys living in the flat file.
Work in progress before deciding to add a template parameter to indexes for the flat file conversion and other traits.
Continue to develop index_base. Rename flat_file_adapter to default_index_traits to reflect full responsibilities. Add NdxTraits parameter to index_set. Change index_unit_test accordingly.