message: Add unsafe add (input|output) path variants
diff: @@ -431,6 +431,13 @@ public:
  return {slot, std::move(mgr)};
  }
  + /// Creates an output path for the given stage without any type checking.
  + template <class Out, class... Ts>
  + output_stream<Out, Ts...> add_unsafe_output_path(stream_manager_ptr mgr) {
  + auto slot = assign_next_pending_slot(mgr);
  + return {0, slot, std::move(mgr)};
  + }
  +
  /// Creates an input path for given stage.
  template <class In, class Result, class Out, class Scatterer, class... Ts>
  make_sink_result<In, Result>
  @@ -440,6 +447,14 @@ public:
  return {slot, std::move(mgr)};
  }
  + /// Creates an input path for given stage.
  + template <class Result, class In>
  + stream_result<Result> add_unsafe_input_path(const stream<In>&,
  + stream_manager_ptr mgr) {
  + auto slot = assign_next_slot(mgr);
  + return {slot, std::move(mgr)};
  + }
  +
  /// Creates a new stream source and starts streaming to `dest`.
  /// @param dest Actor handle to the stream destination.
  /// @param xs User-defined handshake payload.
  