Added `cradle::enable_if_same`.

* jamroot: Removed `--show-progress` option for unit test runner.
* cradle/type_traits/enable_if_same.hpp: New. Added
  `cradle::enable_if_same`.
* test/unit/type_traits/jamfile: Added copyright.
* test/unit/type_traits/enable_if_same_run.cpp: New. Added a unit test of
  `cradle::enable_if_same`.
* test/unit/type_traits/identity_run.cpp: Added copyright.
* test/unit/type_traits/is_callable_run.cpp: Added copyright.
* test/unit/type_traits/remove_rvalue_reference_run.cpp: Added copyright.