UnionConverter: Handle Nested Unions

[bartelink]

Nested unions don't roundtrip; this PR addresses that by removing the inlining of nested objects when a converter is in play

[bartelink]

CC @gardito @fnipo @NickDarvey I can't think of a cleaner way to manage this, thoughts?

[NickDarvey]

You could maybe check that the JsonConverterAttribute's ConverterType is typeof<UnionConverter> just in case someone has a DU with a field of a record type that has a JsonConverterAttribute. I'm not sure how likely that is though.

[bartelink]

@NickDarvey Hm, that's a damn good question. I'm not sure I know the answer so I did b77261f instead.

I tried to extend the check in the way you suggested, but ran aground and then ran out of time :(

An example case my hack does not address is allowing conversions implemented as a JsonIsomorphism that map the value to a record (I added a IsomorphismUnionEncoder example) to benefit from the inlining without having to use that technique top to bottom.

I'm thinking some mix of having tighter rules in #50 and trying to strip down the impl in #43 may be some of the answer.

For now, I need to ship this so its 🙈for the moment...