extensible CAS and CAS extensions

extensible CAS and CAS extensions

  DEFINE-CAS-EXPANDER and DEFCAS are analogous to DEFINE-SETF-EXPANDER and
  DEFSETF, including CAS-functions similar to SETF-functions:

   (defun (cas foo) (old new ...) ...)

  THis is exported from SB-EXT for users to play with, and used to implement
  our CAS places internally.

  Add support for CAS of:

   * SLOT-VALUE

   * STANDARD-INSTANCE-ACCESS

   * FUNCALLABLE-STANDARD-INSTANCE-ACCESS

  In case of SLOT-VALUE we don't yet support any optimizations or specify
  results when SLOT-VALUE-USING-CLASS or friends are in play -- perhaps later
  we can add

    (CAS SLOT-VALUE-USING-CLASS) &co

  in order to support it for arbitrary instances.

  Adding support for permutation vector optimization should not be too hard
  either, but let's let the dust settle first...