Fix a "bug" (difficult to tell if it's really a bug, but if it
isn't we shouldn't be emitting a CONSTANT-MODIFIED warning on
it) in the ctor MAKE-INSTANCE optimization.
... we can't do `(setf (cdr ',place) ...) any more, so instead
wrap another function around it and close over the
locations.
... test case (related to CLASS-13.1 from PFD ansi-tests).