Fix macro crash when handling sealed case classes

[nox213]

Fixes #628

Sealed classes can be instantiated directly. Therefore, when generating the typeTag, I included the sealed class itself. This ensures that the macro correctly handles sealed classes without subclasses. To check whether it’s a sealed class and not a trait or abstract class, I used the following condition:

sealedParents.find(_ == tpe.typeSymbol)

Since trait and abstract class cannot be instantiated, I believe this approach works. However, I could also explicitly check that the symbol is not a trait or an abstract class, if needed.

What do you think?

[lihaoyi]

@nox213 looks great, thanks!