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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[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.