Added flatMapK to FreeT (#3235)

* Added flatMapK to FreeT (where it is useful, unlike with Free where it's simply foldMap)

* Removed mixed bounds syntax

free/src/main/scala/cats/free/FreeT.scala

@@ -32,6 +32,19 @@ sealed abstract class FreeT[S[_], M[_], A] extends Product with Serializable {
         Suspend(mn(m))
     }
 
+  /**
+   * Modify the context `M` using the transformation `mn`, flattening the free
+   * suspensions into the outer.
+   */
+  def flatMapK[N[_]](mn: M ~> FreeT[S, N, *])(implicit S: Functor[S], N: Monad[N]): FreeT[S, N, A] = {
+    def loop(ftft: FreeT[S, FreeT[S, N, *], A]): FreeT[S, N, A] =
+      ftft.resume.flatMap { e =>
+        e.fold(sft => FreeT.liftF[S, N, FreeT[S, FreeT[S, N, *], A]](sft).flatMap(loop(_)), a => FreeT.pure(a))
+      }
+
+    loop(mapK(mn))
+  }
+
   /** Binds the given continuation to the result of this computation. */
   final def flatMap[B](f: A => FreeT[S, M, B]): FreeT[S, M, B] =
     FlatMapped(this, f)