1.0.6.5: potential CLOS GC safety issue

1.0.6.5: potential CLOS GC safety issue

 * EMIT-FETCH-WRAPPER needs to emit code that checks that it has a
   real standard instance (as opposed to a structure) before it can
   pull the slots: if the structure eg. has no slots at all we would
   be pulling garbage into a lisp variable, which is not good (TM),
   though it should be non-serious on GENCGC platforms.

   To make this fast we add a new slot to LAYOUT: FOR-STD-CLASS-P,
   which is always NIL for layouts, and T for wrappers.

 * Remove one redundant SET-DFUN, which may have been needed in long-gone
   days when cache vectors were resourced, but not anymore.