Add Challenge 16: Iterator

[thanhnguyen-aws]

In this PR, I add a challenge for Iterator functions in (library/core/src/iter/adapters)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[celinval]

I'm curious why some of these challenges are focused on correctness.

[carolynzech]

1. You'll need to add links to these challenge files in SUMMARY.md as well.
2. I don't think we should merge any challenges about correctness--this effort is about safety, and we should stay focused on that goal.

[carolynzech]

Thanks! See my comment above about reward amounts; we still shouldn't merge this yet, but on the technical side we're good to go.

[carolynzech]

trigger approval check