Scala 3 Union types don't work well

[lihaoyi]

Overall ticket to supersede the following specific issues:

1. Union Types: work... but could be better supported? #481
2. UPickle changes the meaning of an empty string encoded as a union #478
3. Serialize intersection types? #387

We never had special handling for Scala 3 unions in the past, and so whatever behavior is happening now is somewhat arbitrary, not well defined, and probably not intentional. We should come up with some design for handling these kinds of types

[mio-19]

I would like to suggest implementing ReadWriter for unions when members can be distinguished by java.lang.Class.
Something like

def union2RW[A:ClassTag,B:ClassTag](implicit a: ReadWriter[A], b: ReadWriter[B]): ReadWriter[A | B]

[lihaoyi]

@mio-19 sound reasonable!