Consider adding Json Pattern Functor

@travisbrown wondering if you’d agree to the addition in circe-core of a Pattern Functor version of circe’s Json ADT, which is required when working with recursion-schemes. I’ve been using the code below in a few occurrences, maybe it’d be worth PR-ing ?

sealed trait JsonF[+A]

case object JNullF extends JsonF[Nothing]
case class JBooleanF(b: Boolean) extends JsonF[Nothing]
case class JNumberF(n: JsonNumber)  extends JsonF[Nothing]
case class JStringF(s: String) extends JsonF[Nothing]
case class JArrayF[A](value: Vector[A]) extends JsonF[A]
case class JObjectF[A](fields: Vector[(String, A)]) extends JsonF[A]

object JsonF {
   implicit val jsonTraverse : cats.Traverse[JsonF] = ??? 
   val unfold : Json => JsonF[Json] = ???
   val fold : JsonF[Json] => Json = ???
} 

[travisbrown]

@Baccata Yes, this is something I’ve played with and have been meaning to add for a while—I really wanted to land the circe-algebra work first, but I don’t think that’s necessary. I’d merge a new modules adding this today, and would be happy to discuss where it would fit in core.

[travisbrown]

Done in #1092 (and the follow-up #1094).