Reorganize slynk into its own directory

* slynk/slynk-loader.lisp (sly-version-string): Look for sly.el
one directory up just like slynk/slynk.lisp.
(src-files): No need to add '("lib" "lisp"") to base path.

* sly.el (sly-slynk-path): New function.
(sly-init-using-asdf, sly-init-using-slynk-loader): Use `sly-slynk-path'

* slynk/slynk.asd: Simplify. Contrib systems still a bit hackish.
Forgot to rename swank -> slynk in slynk/backend/slynk-mkcl.lisp
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.
Remove stale files and makefile targets

* .gitattributes: Remove file.

* .gitref: Remove file.

* Makefile (elpa-sly, elpa, dist): Delete stale targets.

* contrib/Makefile (elpa): Delete stale targets.