Added `cradle::is_plain`.

* cradle/type_traits/is_plain.hpp: Added `cradle::is_plain`.
* test/exception/jamfile: Fixed a bug.
* test/type_traits/jamfile: Added a test for `cradle::is_plain`.
* test/type_traits/is_plain_compile.cpp: Added a test for
  `cradle::is_plain`.