Fix a comment in NaturalTransformation

Reported By: Andrej Bauer
Duals -> Dual

For consistency with the singular of [Category], [Functor], etc.

Reported By: Andrej Bauer
Solve trivial obligations after [path_functor]
Port subcategories
Fix ambiguious and incorrect references
Merge pull request #227 from JasonGross/categories3

Port subcategories