Rename common-lisp directory to lisp

I could have renamed common-lisp to common_lisp. I think Nix prefers underscores
to hyphens.

common-lisp/unit-testing.lisp

@@ -1,13 +0,0 @@
-(in-package #:cl-user)
-(defpackage #:my-test
-  (:documentation "Unit testing in Common Lisp.")
-  (:use #:cl))
-(in-package #:my-test)
-
-(plan 3)
-
-(ok (not (find 4 '(1 2 3))))
-(is 4 4)
-(isnt 1 #\1)
-
-(finalize)