Add annotation to omit discriminator for sealed hierarchies

[rallydan]

While the default mechanism of including a discriminator allows for easy parsing, sometimes there is a need to match a specific JSON schema that doesn't include a single field that can be used for this purpose.

We should add an annotation that can be used on the trait that indicates that no discriminator will be present, and still allow generating a From on the trait, and both From and To on the leaf classes. Only macroTo on the base class(es) should fail. Users can then either hand-code a To, or omit it if it is not needed.

@nodiscriminator
sealed trait Fruit

case class Apple(seeds: Int) extends Fruit

implicit val fruitPickler: From[Fruit] = macroFrom
implicit val applePickler: FromTo[Apple] = macroFromTo

val red: Fruit = Apple(15)
val json = FromScala(red).transform(ToJson.string) // """{"seeds":15}"""
val reread = FromJson(json).transform(ToScala[Apple]) // Apple(15)