Method operator co_await

include/exec/task.hpp:525–529

Make this task generally awaitable:

      // Make this task generally awaitable:
      constexpr auto operator co_await() && noexcept -> __task_awaiter<void>
        requires __minvocable_q<awaiter_context_t, __promise>
      {
        return __task_awaiter<void>{std::exchange(__coro_, {})};
      }

      constexpr ~basic_task()
      {