Allow generic sets to be serialized

[zapek]

Currently a Set is considered as EnumSet, but there could be Set or so...

When a set is encountered, we should try to get its type (ParameterizedType has it I think) then use another path if it's not an EnumSet. That way we avoid having to put a special value in the annotation.

On second thought, that only works for deserialize() as it has a Field parameter, serialize() will have to be redesigned somehow (grmbl).