Document some of the concepts used in the library.
Add the EqualityComparable and LessThanComparable concepts.
Move some intrinsics to operators/.
Remove the `intrinsic` namespace, which was a bad idea.
Rename view, algorithm and container groups in the documentation.
Move functional components to functional/.
Remove detail::always_false.
Use `equal_to` instead of `is_same` everywhere possible.
Refactor generic unit tests and intrinsics to fit concepts.