Implement equalp callbacks for smobs.

* src/channel-type.c (equalp_channel): New procedure.
  (init_channel_type): Register the equalp callback.
* src/key-type.c (equalp_key): New procedure.
  (init_key_type): Register the equalp callback.
* src/session-type.c (equalp_session): New procedure.
  (init_session_type): Register the equalp callback.