Cleaning up implementation of fileutils.jsonio MultiTypeSerializer registration

[timbernat]

• Incorporate (Multi)TypeSerializer in the json.JSONEncoder/Decoder paradigm (or at the very least implement encode() (“default()”) and decode() methods)
• Rewrite MultiTypeSerialize decoder to be dict-based (to avoid traversing entire list of encoder/decoders on every translation)
• Make serializer non-initializable from dataclasses decorated by make_jsonifiable()
  • Remove serializer from subclasses dataclass_fields attr