fix another LET*/:interpret bug

reported by Douglas Katzman sbcl-devel 2013-09-08
relax restriction on defstruct slot names

DEFSTRUCT slot names are a bit weird, but behave basically like
strings rather than like symbols.  This makes their interaction with
the MOP a little problematic, but for now simply allow things to
pass through and generally work "as expected".

Report from Eric Marsden sbcl-devel 2013-11-05
partially rewrite accessor-values-internal

It looks old and crufty, but the reason was to handle the admittedly
esoteric case of NIL as a slot name, which can happen in structure
classes.  Be more defensive against merrily finding slot definitions
that have nothing to do with the method in question.

This drive-by fixes lp#633911