Update to SBCL 1.1.18

changes in sbcl-1.1.18 relative to sbcl-1.1.17:
  * optimization: COERCE is now more effecient for more cases when the type is
    known at compile-time.
    (lp#1309815)
  * bug fix: correctly inherit condition initforms. (lp#1300904)
  * bug fix: properly pprint literal functions inside nested lists.
    (lp#1300716)
  * bug fix: more-correctly handle array-type unity (broken for ages, causing
    compilation problems since 1.1.13.x due to smarter TYPEP type propagation,
    reported by jasom in #lisp).

(asau)