Method spawn

include/exec/async_scope.hpp:839–848

      template <__movable_value _Env = env<>, sender_in<__spawn_env_t<_Env>> _Sender>
        requires sender_to<__nest_sender<__decay_t<_Sender>>, __spawn_receiver<_Env>>
      void spawn(_Sender&& __sndr, _Env __env = {})
      {
        using __opstate_t = __spawn_opstate<__nest_sender<__decay_t<_Sender>>, _Env>;
        // this will connect and start the operation, after which the operation state is
        // responsible for deleting itself after it completes.
        [[maybe_unused]]
        auto* __opstate = new __opstate_t{nest(static_cast<_Sender&&>(__sndr)),
                                          static_cast<_Env&&>(__env),
                                          &__impl_};
      }

      template <__movable_value _Env = env<>, sender_in<__env_t<_Env>> _Sender>
      [[nodiscard]]