Method spawn_future

include/exec/async_scope.hpp:851–859

      template <__movable_value _Env = env<>, sender_in<__env_t<_Env>> _Sender>
      [[nodiscard]]
      auto spawn_future(_Sender&& __sndr, _Env __env = {}) -> __future_t<_Sender, _Env>
      {
        using __state_t = __future_state<__nest_sender<__decay_t<_Sender>>, _Env>;
        auto __state    = std::make_unique<__state_t>(nest(static_cast<_Sender&&>(__sndr)),
                                                   static_cast<_Env&&>(__env),
                                                   &__impl_);
        return __future_t<_Sender, _Env>{std::move(__state)};
      }

      [[nodiscard]]
      constexpr auto get_stop_source() noexcept -> inplace_stop_source&