Add PeekableIterator trait

[bluebear94]

Adds the PeekableIterator trait (tracking issue: #132973) and implementations for slice::Iter and ops::Range.

I tried to implement it for Cloned, but I couldn’t get the types and lifetimes to line up, so the unit test at library/core/tests/iter/traits/peekable.rs is not yet compilable.

[rustbot]

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[scottmcm]

Since you said this isn't compilable, I've marked it waiting on author. Please flip it back once it's ready for a review.

Note that if you're having trouble with any particular impl, I'd suggest just skipping it for now. Additional impls can always come later in future PRs; they don't need to be in the first one.

[bluebear94]

Do you think it would be better to remove the trait bound from the PeekedItem associated type altogether?

[bors]

☔ The latest upstream changes (presumably #135937) made this pull request unmergeable. Please resolve the merge conflicts.

[scottmcm]

Looks like this needs some conflicts resolved, at least?

Once this is ready for review, please use @rustbot ready so the bot will update the labels appropriately to bring it back to my radar.

[bluebear94]

Sorry, I haven’t had the motivation to work on this for a long time. I think this needs more thinking on design.