SCM_DEBUG fix: Don't apply SCM_CAR to non-pairs when walking argument

SCM_DEBUG fix: Don't apply SCM_CAR to non-pairs when walking argument
lists in method cache matching.

* libguile/goops.c (scm_mcache_lookup_cmethod): Don't apply SCM_CAR to
  non-pairs when walking argument lists in method cache matching.
  Don't check for CLASSP or symbol in the car slot, since the end of
  the specifier list is a non-pair.  Update comments to reflect new
  structure of method cache entry.
* module/oops/goops/dispatch.scm: Update comments here too.