Added TODO for index_set, etc., close() function. Refined size_t_codec interface to be easily usable by index_traits. Changed the index traits string_view specialization to use byte count for size, rather than relying on '\0' termination. This better meets contract requirements than using a fixed sized count field. '\0' was also inefficient for very long strings and did not allow strings containing '\0' as an element value. Added test cases to index_unit_test.