core: Add DoubleEndedIterator::rfold

rfold is the reverse version of fold.

Fold allows iterators to implement a different (non-resumable) internal
iteration when it is more efficient than the external iteration
implemented through the next method. (Common examples are VecDeque and
.chain()).

Introduce rfold() so that the same customization is available for
reverse iteration. This is achieved by both adding the method, and by
having the Rev<I> adaptor connect Rev::rfold -> I::fold, Rev::fold -> I::rfold.

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