Renamed `cradle::is_plain` to `cradle::is_decayed`.

* cradle/type_traits/is_plain.hpp: Moved to
  `cradle/type_traits/is_decayed.hpp`.
* cradle/type_traits/is_decayed.hpp: Moved from
  `cradle/type_traits/is_plain.hpp`. Renamed `cradle::is_plain` to
  `cradle::is_decayed`.
* test/type_traits/is_plain_compile.cpp: Moved to
  `cradle/type_traits/is_decayed_compile.cpp`.
* test/type_traits/is_decayed_compile.cpp: Moved from
  `test/type_traits/is_plain_compile.cpp`. Renamed `cradle::is_plain` to
  `cradle::is_decayed`.