core: implement FusedIterator for StepBy

[Lfan-ke]

Implements the accepted ACP rust-lang/libs-team#757.

StepBy yields no more items once its underlying iterator is exhausted, so it is fused whenever the underlying iterator is fused. StepBy was added in 1.28.0, just after the batch of FusedIterator impls stabilized in 1.26.0 (Map, Skip, Take, Enumerate, ...), so it was left out.

impl<I: FusedIterator> FusedIterator for StepBy<I> {}

This is insta-stable, matching the sibling adapter impls, so it needs a libs-api FCP.

Motivation from the ACP: crates with traits refined on top of FusedIterator (e.g. range-set-blaze's SortedStarts) cannot cover StepBy today, even though it would always be valid.

A test_step_by_fused regression test is added.

[rustbot]

r? @jhpratt

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Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: libs
• libs expanded to 12 candidates
• Random selection from Darksonn, JohnTitor, Mark-Simulacrum, clarfonthey, jhpratt

[jhpratt]

r=me with completed FCP. Nominating for that purpose.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[madsmtm]

I think this might be a duplicate of #156999?

[Lfan-ke]

Thanks @madsmtm - you're right, #156999 predates this and implements the same ACP (rust-lang/libs-team#757). I wasn't aware of it when I opened this one. This PR has completed the FCP and carries r=me from @jhpratt, so it's ready to land, but I'm equally happy to close it in favour of #156999 if that's preferred - whichever the team would rather merge.

[clarfonthey]

Fixed labels, @rustbot reroll since I believe @jhpratt has no capacity. (Feel free to steal the review back if you're so inclined.)

[clarfonthey]

(Never mind, there is an FCP on the ACP.)

[jhpratt]

r? jhpratt

This has already been reviewed. I looked at everything where I was assigned and rerolled as necessary. If I'm on other PRs, it's by choice :) I appreciate it though!

[clarfonthey]

Yeah, my bad here; I had missed that what was missing was FCP, not review. We were doing triage of outstanding FCPs, too…