message: Add make_continuous_source
diff: @@ -505,6 +505,36 @@ public:
  std::move(pull), std::move(done), std::move(fin), token);
  }
  + /// Creates a new continuous stream source by instantiating the default
  + /// source implementation with `Driver. `The returned manager is not
  + /// connected to any slot and thus not stored by the actor automatically.
  + template <class Driver, class Init, class Pull, class Done,
  + class Finalize = unit_t>
  + typename Driver::source_ptr_type
  + make_continuous_source(Init init, Pull pull, Done done, Finalize fin = {}) {
  + using detail::make_stream_source;
  + auto mgr = make_stream_source<Driver>(this, std::move(init),
  + std::move(pull), std::move(done),
  + std::move(fin));
  + mgr->continuous(true);
  + return mgr;
  + }
  +
  + /// Creates a new continuous stream source by instantiating the default
  + /// source implementation with `Driver. `The returned manager is not
  + /// connected to any slot and thus not stored by the actor automatically.
  + template <class Init, class Pull, class Done, class Finalize = unit_t,
  + class DownstreamManager = broadcast_downstream_manager<
  + typename stream_source_trait_t<Pull>::output>>
  + stream_source_ptr<DownstreamManager>
  + make_continuous_source(Init init, Pull pull, Done done, Finalize fin = {},
  + policy::arg<DownstreamManager> = {}) {
  + using driver = detail::stream_source_driver_impl<DownstreamManager, Pull,
  + Done, Finalize>;
  + return make_continuous_source<driver>(std::move(init), std::move(pull),
  + std::move(done), std::move(fin));
  + }
  +
  template <class Driver, class... Ts>
  make_sink_result<typename Driver::input_type>
  make_sink(const stream<typename Driver::input_type>& src, Ts&&... xs) {
  