Precisely,
1. Rename hana::_type to hana::basic_type, and document it
2. Rename hana::type<> to hana::type_c<>
3. Create and document the hana::type<> type
- Allow specializing models_impl with a forward declaration only
- Modularize remaining concept headers
- Include <boost/hana.hpp> in unit tests to ease the transition
- Add the core/dispatch.hpp header to reduce the number of boilerplate includes
