Fix ClassCastException on import given

[lihaoyi]

Fixes #469

The basic problem is that superTypeReader and superTypeWriter were triggering for Types T and V which were identical, meaning it was getting used even when not necessary due to import upickle.default.given increasing its implicit priority.

This PR adds a : SumOf implicit requirement to V, ensuring that superTypeReader and superTypeWriter only trigger when V is a parent sealed trait/class/enum type, won't trigger when T =:= V as a case class, and thus the .asInstanceOf[TaggedReader[T]] cast is valid