Implement package-local nicknames

With package-local nicknames now being present in almost all of the free software Lisp world (SBCL, ECL, CCL, Clasp, ABCL), the only alive free CL implementation I am aware of that does not yet implement those is CLISP. It would therefore be beneficial to implement the PLN extension in CLISP to allow all programmers working with free implementations to leverage them.