adding helper method toFreeT to Free (#2724)

* Free should able to convert to FreeT

* refactor

* fix format with sbt freeJVM/scalafmt

* mend test format

* address comments

* no need to specify freet monad instance

* one liner

free/src/main/scala/cats/free/Free.scala

@@ -30,7 +30,7 @@ sealed abstract class Free[S[_], A] extends Product with Serializable {
   final def mapK[T[_]](f: S ~> T): Free[T, A] =
     foldMap[Free[T, ?]] { // this is safe because Free is stack safe
       λ[FunctionK[S, Free[T, ?]]](fa => Suspend(f(fa)))
-    }(Free.catsFreeMonadForFree)
+    }
 
   /**
    * Bind the given continuation to the result of this computation.
@@ -183,6 +183,9 @@ sealed abstract class Free[S[_], A] extends Product with Serializable {
   final def inject[G[_]](implicit ev: InjectK[S, G]): Free[G, A] =
     mapK(λ[S ~> G](ev.inj(_)))
 
+  final def toFreeT[G[_]: Applicative]: FreeT[S, G, A] =
+    foldMap[FreeT[S, G, ?]](λ[S ~> FreeT[S, G, ?]](FreeT.liftF(_)))
+
   override def toString: String =
     "Free(...)"
 }

free/src/test/scala/cats/free/FreeSuite.scala

@@ -83,10 +83,9 @@ class FreeSuite extends CatsSuite {
     fa should ===(Free.pure[Option, Int](n))
   }
 
-  test("foldMap is stack safe") {
-    trait FTestApi[A]
-    case class TB(i: Int) extends FTestApi[Int]
-
+  trait FTestApi[A]
+  case class TB(i: Int) extends FTestApi[Int]
+  object FTestApi {
     type FTest[A] = Free[FTestApi, A]
 
     def tb(i: Int): FTest[Int] = Free.liftF(TB(i))
@@ -100,8 +99,14 @@ class FreeSuite extends CatsSuite {
     def runner: FunctionK[FTestApi, Id] = λ[FunctionK[FTestApi, Id]] {
       case TB(i) => i + 1
     }
+  }
+
+  test("foldMap is stack safe") {
+    assert(10000 == FTestApi.a(0).foldMap(FTestApi.runner))
+  }
 
-    assert(10000 == a(0).foldMap(runner))
+  test("toFreeT is stack-safe") {
+    FTestApi.a(0).toFreeT[Id].foldMap(FTestApi.runner) should ===(FTestApi.a(0).foldMap(FTestApi.runner))
   }
 
   test(".runTailRec") {