Make nicer primitive functions for car/cdr

If it isn't a CONS, we return NIL instead of failing.  This way, we
can use it in our general iteration functions.  Eventually the actual
runtime would use this as well.  The macros are mostly for internal
use to do assignment etc.

alisp.h

@@ -126,6 +126,9 @@ vec_t *as_vec(lisp_t *);
 #define CAR(L) (as_cons(L)->car)
 #define CDR(L) (as_cons(L)->cdr)
 
+lisp_t *car(lisp_t *);
+lisp_t *cdr(lisp_t *);
+
 /// Pointer tagging scheme for lisps
 
 typedef enum Tag