1.0.4.85: small PCL cleanups and thread-safety notes

1.0.4.85: small PCL cleanups and thread-safety notes

 * CACHE-NUMBER-VECTOR-REF and WRAPPER-CACHE-NUMBER-VECTOR-REF were
   equivalent to LAYOUT-CLOS-HASH. Use only the latter for easier
   reading.

 * *PREVIOUS-NWRAPPERS* is protected by the PCL lock, so ok.

 * The whole of dlisp2.lisp is currently dead code.