Make the Properties class more type-safe.
Ditch returning the "assigned key" from the Properties map. Just return originals.
Make the Property class type.
Split `Key` into a `Key` interface and `AbstractKey` abstract class.
Return a `ValidRekord` from `ValidatingRekord::fix`.
Rename `FixedRekord::containsKey` to `has`.
Ensure `Rekord::has` returns the correct value for a transformed key.