Add @Wrapper annotation

[alexarchambault]

To annotate case classes having a single field, like

@Wrapper case class Something(underlying: Underlying)

whose JSON representation is the one of the field.

Would avoid to write boilerplate like

case class Something(underlying: String)
object Something {
  implicit val decode = DecodeJson.of[Underlying].map(Something(_))
  implicit val encode = EncodeJson.of[Underlying].contramap[Something](_.underlying)
}

for simply

@Wrapper case class Something(underlying: Underlying)

[tylerjharden]

Maybe as a macro?