Rename common-lisp directory to lisp

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

common-lisp/main.lisp

@@ -1,15 +0,0 @@
-(in-package #:cl-user)
-(defpackage #:utils
-  (:documentation "Some utility functions and macros to wet my beak.")
-  (:use #:cl)
-  (:shadow #:type))
-(in-package #:utils)
-
-(defmacro type (name in out)
-  `(declaim (ftype (function ,in ,out) ,name)))
-
-(defmacro comment (&rest _forms) nil)
-
-(type add (int int) int)
-(defun add (a b)
-  (+ a b))