Allow querying whether a type models a concept

[ldionne]

Something like te::models<Concept, T> would be neat. That would require getting the concept map and checking whether all the methods are fulfilled.

[vittorioromeo]

Is this basically asking to refactor and expose

constexpr bool all_functions_satisfied = decltype(boost::hana::is_subset(
      boost::hana::keys(Concept::all_clauses()),
      boost::hana::keys(merged)
    ))::value;

from concept_map.hpp:240?

[ldionne]

Yes, pretty much. I still need to figure out how I want to tie this to the rest of the library from a usage perspective.