rforEach(fn, thisp = this) {
        for (const i of this.#rindexes()) {
            const v = this.#valList[i];
            const value = this.#isBackgroundFetch(v)
                ? v.__staleWhileFetching
                : v;
            if (value === undefined)
                continue;
            fn.call(thisp, value, this.#keyList[i], this);
        }
    }
The same as {@link LRUCache.forEach} but items are iterated over in
reverse order.  (ie, less recently used items are iterated over first.)