lisp: split off lisp_free as it's own function

lisp_free will do a shallow clean of any object, freeing its
associated memory.  It won't recur through any containers, nor will it
freakout if you give it something that is constant (symbols, small
integers, NIL, etc).

include/alisp/lisp.h

@@ -49,6 +49,8 @@ vec_t *as_vec(lisp_t *);
 lisp_t *car(lisp_t *);
 lisp_t *cdr(lisp_t *);
 
+void lisp_free(lisp_t *);
+
 #endif
 
 /* Copyright (C) 2026 Aryadev Chavali