Audit the `iter_` methods in `nimbus-fml`

[jhugman]

a pass by thought about the return types, a "good first bug" could be to turn the Vecs into iterators in any iter_* functions 🤔 (I know this isn't the only one, so not requesting changes on it here)

Originally posted by @tarikeshaq in https://github.com/mozilla/application-services/pull/4999#discussion_r899742398

┆Issue is synchronized with this Jira Task