Better iterator ergonomics for `Pushes` and `SettledOrders`

[fedgiac]

There was quite a lot of discussion on how it's best to implement iterators for Pushes and SettledOrders.
See for example:

• Suggestion: Simplify SettledOrders IntoIterator type #45
• Parse pushes in FinalizeSettle #47 (comment)
• Parse pulls in BeginSettle #38 (comment) Parse pulls in BeginSettle #38 (comment)
• https://nomevlabs.slack.com/archives/C0980HAC78S/p1783068081162569

It's unclear to me what's the best way to do it, but I'm open to different solutions.
This issue is to collect discussion and keep it separate from other features.