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.

constructor.c

@@ -46,3 +46,19 @@ lisp_t *intern(sys_t *sys, sv_t sv)
   char *str = sym_table_find(&sys->symtable, sv);
   return tag_sym(str);
 }
+
+lisp_t *car(lisp_t *lsp)
+{
+  if (!IS_TAG(lsp, CONS))
+    return NIL;
+  else
+    return CAR(lsp);
+}
+
+lisp_t *cdr(lisp_t *lsp)
+{
+  if (!IS_TAG(lsp, CONS))
+    return NIL;
+  else
+    return CDR(lsp);
+}