Replace generators with IteratorIterator in custom iterators

[alcaeus]

Feature Request

Q	A
New Feature	improvement
RFC	no
BC Break	no

Summary

The HydratingIterator and CachingIterator classes use generators to wrap the given traversable. As described in #2092 and fixed in #2100, this can leak memory if the iterator class is destroyed while the generator isn't exhausted completely.

IteratorIterator can be used instead, but since there are some changes to generators with respect to rewind, the change shouldn't be done in a patch release.

[soyuka]

What's the status on this @alcaeus ?

[alcaeus]

The status is the same as five years ago, except that I have no recollection of what the issue was 🤦‍♂️

Did you encounter an issue around iterators that needs addressing?

[soyuka]

I think we should close this, I'll open a new one if I can reproduce!

[alcaeus]

No complaints from me 😊