1.0.5.46: improve handling of non-standard subclasses of SB-MOP:SPECIALIZER

1.0.5.46: improve handling of non-standard subclasses of SB-MOP:SPECIALIZER
	... define SPECIALIZER-CLASS-OR-NIL for use in RAISE-METATYPE,
	and adjust RAISE-METATYPE to handle NIL return values.
	... add commentary around RAISE-METATYPE to explain what all the
	metatypes actually mean.
	... EMIT-FETCH-WRAPPER was missing a CONDITION-INSTANCE case,
	and further drew fine distinctions where there were none...
	... so delete BUILT-IN-OR-STRUCTURE-WRAPPER, and call WRAPPER-OF
	instead.  (But leave in the GC safety bug reported sbcl-devel
	2007-05-10.)
	... one more fix to PARAMETER-SPECIALIZER-DECLARATION-IN-DEFMETHOD
	for CLASS-EQ specializers on built-in-classes.