Untagged Variant serialization design discussion

[Waridley]

#743 was scoped down to only including core types and VariantDispatch due to some uncertainty about how corner cases of plain Variants, VariantArrays, and Dictionarys should be handled. This issue is to sort out those decisions.

I will add more detail later, but I wanted to open this to provide a place for the discussion and to gather examples of use cases to inform the decisions.