0.8.16.43: Fixes for various CLOS/MOP bugs

0.8.16.43: Fixes for various CLOS/MOP bugs
            * Correct canonization of DEFCLASS slot options (also move
               checking to macroexpansion time of DEFCLASS, and do
               some trivial reorganization of the defclass.lisp to
               make the diff bigger then it really is.) (reported
               by Bruno Haible)
            * (SETF FIND-CLASS) with new-value that is a
               FORWARD-REFERENCED-CLASS. (reported by Bruno Haible)
            * CLASS-PROTOTYPE signals an error if the class is not yet
               finalized; also clean up some of the class-prototype
               machinery a bit. (reported by Bruno Haible)
            * ALLOCATE-INSTANCE for BUILT-IN-CLASS now a) exists b)
               signals an error.
            * Add tests & record an as-of-yet-unfixed bug.