forked from microsoft/GSL
fixes: https://github.com/microsoft/GSL/issues/1129 * create gsl::swap<T>(T&, T&) which wraps std::swap * specialize gsl::swap<T>(gsl::not_null<T>&, gsl::not_null<T>&) * add tests
fixes: https://github.com/microsoft/GSL/issues/1129 * create gsl::swap<T>(T&, T&) which wraps std::swap * specialize gsl::swap<T>(gsl::not_null<T>&, gsl::not_null<T>&) * add tests