message: Add {in,out}bound_stream_slot forward declarations
diff: @@ -22,6 +22,7 @@ template <class> class callback;
  template <class> class dictionary;
  template <class> class downstream;
  template <class> class expected;
  +template <class> class inbound_stream_slot;
  template <class> class intrusive_cow_ptr;
  template <class> class intrusive_ptr;
  template <class> class optional;
  @@ -56,6 +57,7 @@ template <class, class, class> class broadcast_downstream_manager;
  template <class...> class const_typed_message_view;
  template <class...> class cow_tuple;
  template <class...> class delegated;
  +template <class, class...> class outbound_stream_slot;
  template <class...> class result;
  template <class...> class typed_actor;
  template <class...> class typed_actor_pointer;
  