fix `anyeltypedual(::Union)` and a few minor cleanups

[oscardssmith]

Checklist

• [ ] Appropriate tests were added
• [ ] Any code changes were done in a way that does not break public API
• [ ] All documentation related to code changes were updated
• [ ] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• [ ] Any new documentation only uses public API

Additional context

While I was looking at this, I broadened a few Set and Array to AbstractSet and AbstractArray to better support non-base types.