Implement FusedIterator trait on StepBy if the underlying iterator is fused

[asder8215]

This PR implements the FusedIterator trait for StepBy if the underlying iterator also implements FusedIterator as per accepted in this ACP.

This would have to be insta-stable as we're implementing a trait on a stable feature.

[rustbot]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

Let's try the new rfcbot functionality...

@rfcbot fcp merge libs-api

[rust-rfcbot]

@Mark-Simulacrum has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @the8472

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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.