Builtin function argument parsing

src/stack.h

@@ -30,6 +30,8 @@ struct LispStack {
     size_t first_clear_local_refs; // index of the first frame that has local
                                    // refs that has not been grown
     struct StackFrame *frames;
+
+    LispVal *nogc_retval;
 };
 
 extern struct LispStack the_stack;