Add `.rfind()` to the `Iterator` trait


Since we have both .position() and .rposition(), it is reasonable to add .rfind() as well. For str, we also have both .find() and .rfind().


I think it’s reasonable to add both rfind and rfold and maybe more. One benefit is that the reversed versions cross over with Rev<I> (reversed iterators); Rev<I>::find calls I's rfind and so on.