Tidy up parts of DEFSTRUCT's implementation.

- CLASS-METHOD-DEFINITIONS had nothing to do with MAKE-LOAD-FORM,
  nor was it apparent why the LAYOUT-PURE option should be affected there.

- Source code of :PRINT-{FUNCTION,OBJECT} methods should not be immortalized.

- Alternate metaclass structs don't need to invoke *DEFSTRUCT-HOOKS*

- There are no longer "stereotyped closures" as accessors
Remove :STACKP option to DEFINE-COLD-FOP