Explicitly name types to get better error messages
Extend classes, not objects
Consolidate protocols; remove hack in moldfind

-Merged the IValidateable, IParseable, and IShowable protocols
into IUIType.
- Changed Mold.show to always allow fields that start with __
This allowed removing a hack we had in moldfind to add the
output fields to a display mold
Make defmold take opts optionally