Improve printing of Guile-SSH objects

* ssh/session-type.c (print_session): New procedure.
  (init_session_type): Register `print_session' callback.
* ssh/channel-type.c (print_channel): Print object address.
* ssh/message-type.c (print_message): Likewise.
* ssh/key-type.c (print_key): Likewise.
* ssh/common.c (_scm_object_hex_address): New procedure.
* ssh/common.h (_scm_object_hex_address): Likewise.
* NEWS: Update.