Added `cradle::enable_if_volatile`.

* cradle/type_traits/enable_if_volatile.hpp: Added
  `cradle::enable_if_volatile` and `cradle::disable_if_volatile`.
* test/unit/type_traits/enable_if_volatile_run.cpp: Added unit tests for
  `cradle::enable_if_volatile` and `cradle::disable_if_volatile`.