Implement `DoubleEndedIterator` for `FilterOk`

[Xenira]

Should be doable with rfind unless there is a reason I overlooked.

Would be willing to implement this.

[Philippe-Cholet]

FlattenOk, MapSpecialCase (for MapOk, MapInto) are double ended and ProcessResults since a few weeks.
FilterOk and FilterMapOk are not (yet) but it seems reasonable to me.

Specialization tests and benchmarks will need to be updated but that's simple to do.

[Xenira]

Like that? Have to admit, not entirely comprehending the test system yet.

Also not sure, if rfold needs custom impl. FlattenOk didn't have any. Edit: nvm, found it. So prob. yes.

[Philippe-Cholet]

@Xenira I did not notice before they were not double ended so thanks for this.