Add Serialization for protobuf enum values

This adds another Serialization impl, for the actual enum values (not the enum value descriptors, like ProtobufEnumSerialization).

I used reflection to get at the enum classes "valueOf". Since I only get the method once, I hope it does not add terrible overhead.

By the way, ProtobufEnumSerialization seems incomplete — deserialize returns null. Do we also want serialization for the enum value descriptors? I guess the motivation was that getField for an enum field returns a value descriptor instead of the actual value?