message: Re-organize and complement forward declarations
diff: @@ -67,52 +67,52 @@ template <class, class...> class output_stream;
  // -- classes ------------------------------------------------------------------
  +class abstract_actor;
  +class abstract_group;
  class actor;
  -class error;
  -class group;
  -class message;
  -class node_id;
  -class behavior;
  -class duration;
  -class resumable;
  class actor_addr;
  -class actor_pool;
  -class message_id;
  -class serializer;
  class actor_clock;
  -class actor_proxy;
  -class local_actor;
  -class ref_counted;
  +class actor_companion;
  class actor_config;
  +class actor_control_block;
  +class actor_pool;
  +class actor_proxy;
  +class actor_registry;
  class actor_system;
  +class actor_system_config;
  +class behavior;
  +class blocking_actor;
  class deserializer;
  +class downstream_manager;
  +class downstream_manager_base;
  +class duration;
  +class error;
  +class event_based_actor;
  +class execution_unit;
  +class forwarding_actor_proxy;
  +class group;
  class group_module;
  -class message_view;
  -class scoped_actor;
  class inbound_path;
  -class outbound_path;
  -class abstract_actor;
  -class abstract_group;
  -class actor_registry;
  -class blocking_actor;
  -class execution_unit;
  -class proxy_registry;
  -class stream_manager;
  -class actor_companion;
  +class local_actor;
  class mailbox_element;
  +class message;
  class message_builder;
  class message_handler;
  -class scheduled_actor;
  +class message_id;
  +class message_view;
  +class node_id;
  +class outbound_path;
  +class proxy_registry;
  +class ref_counted;
  class response_promise;
  -class event_based_actor;
  +class resumable;
  +class scheduled_actor;
  +class scoped_actor;
  +class serializer;
  +class stream_manager;
  class type_erased_tuple;
  class type_erased_value;
  -class downstream_manager;
  -class actor_control_block;
  -class actor_system_config;
  class uniform_type_info_map;
  -class forwarding_actor_proxy;
  -class downstream_manager_base;
  // -- structs ------------------------------------------------------------------
  