Rename common-lisp directory to lisp

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

lisp/unit-testing.lisp

@@ -0,0 +1,13 @@
+(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)