Revert to again supply the reverse_endianness_in_place template. This has the effect of providing a default reverse_endianness_in_place for user-defined types. Add error checking for UDT's to verify this works as desired. Provide a note in the conversion docs to that effect.
Drop the _unless_big and _unless_little functions. KISS. They were confusing, and less confusing names were never found.