message: Add missing forward declarations for byte and span
diff: @@ -43,6 +43,7 @@ template <class> class intrusive_ptr;
  template <class> class optional;
  template <class> class param;
  template <class> class serializer_impl;
  +template <class> class span;
  template <class> class stream;
  template <class> class stream_sink;
  template <class> class stream_source;
  @@ -66,7 +67,6 @@ template <class, class, class> class broadcast_downstream_manager;
  template <class...> class cow_tuple;
  template <class...> class delegated;
  template <class...> class result;
  -template <class...> class result;
  template <class...> class typed_actor;
  template <class...> class typed_actor_pointer;
  template <class...> class typed_event_based_actor;
  @@ -173,6 +173,7 @@ config_option make_config_option(T& storage, string_view category,
  // -- enums --------------------------------------------------------------------
  enum class atom_value : uint64_t;
  +enum class byte : uint8_t;
  enum class sec : uint8_t;
  enum class stream_priority;
  