Add iter::reverse_if helper

[nightkr]

Description

This is an alternative to

if cond {
    // must collect here, since foo.rev() is a distinct type from foo
    foo.rev().collect::<Vec<_>>()
} else {
    foo.collect()
}

That is both much more convenient to use and avoids the buffering and allocation.

Definition of Done Checklist

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# Author

# Reviewer
- [ ] Code contains useful comments
- [ ] (Integration-)Test cases added
- [ ] Documentation added or updated
- [ ] Changelog updated
- [ ] Cargo.toml only contains references to git tags (not specific commits or branches)

# Acceptance
- [ ] Feature Tracker has been updated
- [ ] Proper release label has been added

[NickLarsenNZ]

The impl looks good to me. I am curious about two things:

• how this came to be (where will it be used)
• what does marble mean in the tests?

[nightkr]

how this came to be (where will it be used)

https://github.com/stackabletech/hdfs-operator/blob/808f926dc0ad585bac972b0817f1a8548f524725/rust/operator-binary/src/hdfs_controller.rs#L334-L344

what does marble mean in the tests?

The style of black-box testing iterator/async implementation as "this sequence of inputs should generate this sequence of outputs". I originally got it from Rx (where it's both a helpful education tool and a testing framework).