Get rid of when-let as it conflicts with a definition subr-x.el

* slime.el (when-let): Deleted. Update all uses accordingly.
Fix last commit.
Use one package per backend to make interfaces more explicit.

* swank-abcl.lisp, swank-allergo.lisp, swank-ccl.lisp,
swank-clisp.lisp, swank-ecl.lisp, swank-lispworks,
swank-sbcl.lisp, swank-scl.lisp, swank-source-file-cache.lisp,
swank-source-path-parser.lisp: Use packages.
* swank-backend.lisp: Export some more stuff.
Clear all tests out at the beginning.

* slime-tests.el (slime-tests--undefine-all): New function.
* Makefile (compile-swank, run-swank): New targets.

(clean-fasls): Also remove ~/.slime/fasl/.
* Makefile (compile-swank, run-swank): New targets.

(clean-fasls): Also remove ~/.slime/fasl/.