0.9.16.24: (SETF INFO) compiler macro enabled

0.9.16.24: (SETF INFO) compiler macro enabled
 * Since we now support SETF compiler-macros. Unfortunately the
   definition comes too late to affect large parts of SBCL -- but the
   same applies to the venerable compiler-macro for INFO.
 * Use KEYWORDP instead of CONSTANTP in these macros, as that (or
   self-evaluating-p) is what is actually ment -- no EVAL or
   CONSTANT-FORM-VALUE in sight.