Also move slynk-util.lisp out of contribs

* slynk/slynk.asd (:slynk-util): Point system to new file.

* slynk/slynk.asd (:slynk-asdf, :slynk-hyperdoc, :slynk-clipboard)
(:slynk-clipboard):

* slynk/slynk-util.lisp: Moved from contrib/slynk-util.lisp.

* slynk/slynk-loader.lisp: *SOURCE-DIRECTORY* is in *LOAD-PATH* by
  default.