Add hooks to the runtime system that are for use by SOS:

Add hooks to the runtime system that are for use by SOS:

* Add a slot to record types to hold the class wrapper so that records
  can have classes associated with them, and thus be dispatched on by
  generic procedures.

* Maintain a population of record types so that classes can be added
  to records retroactively when the object system is loaded.

* Add a new unparser hook that overrides the default unparser for
  records that satisfy RECORD? (records with explicit unparsers are
  unaffected).

* Add a new unparser hook that may override the representation of
  procedures, so that generic procedures can have a special
  representation.

* Change the DEFINE-STRUCTURE macro so that record-based structures
  don't have an explicit unparser unless the PRINT-PROCEDURE option is
  used.