To supplement the new custom serialization feature, we also would like
the ability to use standard reflection-based serialization, but also to
add some extra data in on the side. This is now possible as auxiliary
serialized data. A class can define auxiliary data serializers which will
add additional data to the serialized record generated by the usual
reflection-based serializers.
Previously when enum values not defined in the original enum type were
used in serialized objects, FC threw exceptions when it encountered
these values. Now it just serializes unknown values to and from numbers
instead of names.
As an added benefit, the compiler now warns you if a value is defined in
the enum, but not reflected!
