Add Coproduct fold (fixes #987)

core/src/main/scala/cats/data/Coproduct.scala

@@ -1,6 +1,7 @@
 package cats
 package data
 
+import cats.arrow.NaturalTransformation
 import cats.functor.Contravariant
 
 /** `F` on the left and `G` on the right of [[Xor]].
@@ -57,6 +58,8 @@ final case class Coproduct[F[_], G[_], A](run: F[A] Xor G[A]) {
   def toValidated: Validated[F[A], G[A]] =
     run.toValidated
 
+  def fold[H[_]](f: NaturalTransformation[F, H], g: NaturalTransformation[G, H]): H[A] =
+    run.fold(f.apply, g.apply)
 }
 
 object Coproduct extends CoproductInstances {
@@ -78,7 +81,6 @@ object Coproduct extends CoproductInstances {
   def left[G[_]]: CoproductLeft[G] = new CoproductLeft[G]
 
   def right[F[_]]: CoproductRight[F] = new CoproductRight[F]
-
 }
 
 private[data] sealed abstract class CoproductInstances3 {