!str #15236 Replace Transformer with Stage

* replace all existing Transformer with Stage (PushPullStage)
* use Flow[ByteString, ByteString] as encoder/decoder transformer in http
* use the IteratorInterpreter for strict if possible
* emit then become
* emit then finish
* termination emits
* FlowTransformerSpec
* rework types to work with Java API
* rename and move things
* add scaladoc

akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowGraphCompileSpec.scala

@@ -3,10 +3,11 @@
  */
 package akka.stream.scaladsl
 
-import akka.stream.{ OverflowStrategy, Transformer }
+import akka.stream.OverflowStrategy
 import akka.stream.FlowMaterializer
 import akka.stream.testkit.AkkaSpec
 import akka.stream.testkit.StreamTestKit.{ PublisherProbe, SubscriberProbe }
+import akka.stream.stage._
 
 object FlowGraphCompileSpec {
   class Fruit
@@ -18,9 +19,10 @@ class FlowGraphCompileSpec extends AkkaSpec {
 
   implicit val mat = FlowMaterializer()
 
-  def op[In, Out]: () ⇒ Transformer[In, Out] = { () ⇒
-    new Transformer[In, Out] {
-      override def onNext(elem: In) = List(elem.asInstanceOf[Out])
+  def op[In, Out]: () ⇒ PushStage[In, Out] = { () ⇒
+    new PushStage[In, Out] {
+      override def onPush(elem: In, ctx: Context[Out]): Directive =
+        ctx.push(elem.asInstanceOf[Out])
     }
   }