- write interface file

- more / better unit tests

- make field / widget registry stuff aware of centralized AT field and widget
  registry

- move 'validators' into field settings area and use a validators registry

- move 'required' and 'create index' options into field settings area

- allow for separate custom hooks for field settings, widget settings,
  storage settings, and general settings; field, widget, and storage custom
  hooks should be either specific to a type or for ALL types
