Auto merge of #35559 - frewsxcv:slice-iter-as-ref, r=alexcrichton

Auto merge of #35559 - frewsxcv:slice-iter-as-ref, r=alexcrichton

Implement `AsRef<[T]>` for `std::slice::Iter`.

`AsRef` is designed for conversions that are "cheap" (as per
the API docs). It is the case that retrieving the underlying
data of `std::slice::Iter` is cheap. In my opinion, there's no
ambiguity about what slice data will be returned, otherwise,
I would be more cautious about implementing `AsRef`.