Add syntax for separate from MonadCombine.

I based myself on the structure in syntax.bitraverse for the two implicit conversions (the
existing for unite and the new for separate).

MonadCombine separate was added in typelevel#864 by @adelbertc.

core/src/main/scala/cats/syntax/monadCombine.scala

@@ -1,21 +1,26 @@
 package cats
 package syntax
 
-trait MonadCombineSyntax {
+trait MonadCombineSyntax extends MonadCombineSyntax1 {
   // TODO: use simulacrum instances eventually
-  implicit def nestedMonadCombineSyntax[F[_]: MonadCombine, G[_], A](fga: F[G[A]]): NestedMonadCombineOps[F, G, A] =
-    new NestedMonadCombineOps[F, G, A](fga)
+  implicit def monadCombineSyntax[F[_]: MonadCombine, G[_], A](fga: F[G[A]]): MonadCombineOps[F, G, A] =
+    new MonadCombineOps[F, G, A](fga)
 }
 
-final class NestedMonadCombineOps[F[_], G[_], A](fga: F[G[A]])(implicit F: MonadCombine[F]) {
+private[syntax] trait MonadCombineSyntax1 {
+  implicit def nestedMonadCombineSyntax[F[_]: MonadCombine, G[_, _], A, B](fgab: F[G[A, B]]): NestedMonadCombineOps[F, G, A, B] =
+    new NestedMonadCombineOps[F, G, A, B](fgab)
+}
+
+final class MonadCombineOps[F[_], G[_], A](fga: F[G[A]])(implicit F: MonadCombine[F]) {
 
   /**
    * @see [[MonadCombine.unite]]
    *
    * Example:
    * {{{
    * scala> import cats.std.list._
-    * scala> import cats.std.vector._
+   * scala> import cats.std.vector._
    * scala> import cats.syntax.monadCombine._
    * scala> val x: List[Vector[Int]] = List(Vector(1, 2), Vector(3, 4))
    * scala> x.unite
@@ -24,3 +29,21 @@ final class NestedMonadCombineOps[F[_], G[_], A](fga: F[G[A]])(implicit F: Monad
    */
   def unite(implicit G: Foldable[G]): F[A] = F.unite(fga)
 }
+
+final class NestedMonadCombineOps[F[_], G[_, _], A, B](fgab: F[G[A, B]])(implicit F: MonadCombine[F]) {
+
+  /**
+   * @see [[MonadCombine.separate]]
+   *
+   * Example:
+   * {{{
+   * scala> import cats.data.Xor
+   * scala> import cats.std.list._
+   * scala> import cats.syntax.monadCombine._
+   * scala> val l: List[Xor[String, Int]] = List(Xor.right(1), Xor.left("error"))
+   * scala> l.separate
+   * res0: (List[String], List[Int]) = (List(error),List(1))
+   * }}}
+   */
+  def separate(implicit G: Bifoldable[G]): (F[A], F[B]) = F.separate(fgab)
+}