Prefer AsyncIterable over AsyncIterator to reduce errors related to consumers that share iterators

This mitigates some of the issues created by async protocol errors where a single iterator could end up being doubly referenced. It also goes some way to rendering multi() as obsolete..

Also:

• Fix case where a removed node has been re-added (but was treated as removed)
• Apply the same robustness fix from 0.18.7 to filterMap, merge and combine

Full Changelog: https://github.com/MatAtBread/AI-UI/compare/Release-v0.18.7..Release-v0.19.0