Moved `cradle::invoke`.

* cradle/functional/invoke.hpp: Delegated the implementation of
  `cradle::invoke` to `cradle/type_traits/is_callable.hpp`
* cradle/type_traits/is_callable.hpp: Integrated well-formedness tests and
  the implementation of `cradle::invoke`.
Added `cradle::is_callable`.

* cradle/type_traits/is_callable.hpp: Newly added.
* test/type_traits/is_callable_compile.cpp: Newly added.
Merge pull request #2 from Cryolite/feature/is_callable

Add `cradle::is_callable` metafunction