Kill PolymorphicModelResource, implement $ref resolution in field validation ..

- PolymorphicModelResource removed because it is out of scope
- $ref are now resolved in Raw.validate(). That makes it a lot easier to validate complex field types ToX fields.
- fields.Numeric() now has minimum/maximum options
- fields.KeyValue() now with optional 'key_pattern' regex pattern

More work required refactoring resource fields and schemas as they are now accessed from all over the place.