Forward more iterator methods for element iterators

[bluss]

The Iter and IterMut are composed of a contiguous case and a general case, and because of this we should forward most (non-adaptor) methods to the underlying iterators. The existing fold and rfold methods serve as examples. No new custom implementations of the methods are needed in ElementsBase or Baseiter.

The ultimate ones to forward are try_fold and try_rfold, but they need to wait for having stable signatures (all traits in the signature).

Good ones to forward:

• nth
• collect
• all, any
• find, find_map

Some don't matter because they use fold that we already forward: count, max, min, max_by etc, sum, product

[andrei-papou]

Hi @bluss . I'd like to pick this one up.

[jturner314]

@andrei-papou That would be great.

Regarding with methods to forward, @bluss has provided some good suggestions. If you'd like to investigate yourself, I recommend looking at the methods std::slice::Iter overrides and, since we can't yet forward try_fold and try_rfold, the methods of Iterator and DoubleEndedIterator that are implemented in terms of try_fold and try_rfold.