TODO:

  - Implement separate readers, writers, and accessors for attributes
  - Implement predicate for attributes
  - Implement clearer for attributes
  - Implement is => 'rwp' for attributes
  - Implement lazy attributes
  - Implement builder for attributes
  - Implement isa for attributes
  - Implement coerce for attributes
  - Implement triggers for attributes?
  - Implement BUILD and BUILDARGS
