Implement Control#add_to_resource

Also contains a shitton of refactoring, including merging Attributes and
AttributeDefaults, and dropping the exceptional "subresources" behavior
for CollectionResource.
Mixed bag

- Add initial implementation of the Halo format (untested)
- Bring mutation back to 100% (excluding Yaks::Format::Halo which isn't
  loaded yet, and Yaks::Format::HTML, which is in a separate gem)
- Finishing touches on the form controls implementation
- Some more refactoring, especially regarding Yaks::Attributes