Introduces fold as a Flow transformation and generalizes Sink.fold to be Flow.fold + Sink.head

Conflicts:
	akka-stream/src/main/scala/akka/stream/impl/ActorMaterializerImpl.scala
	akka-stream/src/main/scala/akka/stream/impl/Stages.scala
	akka-stream/src/main/scala/akka/stream/scaladsl/Sink.scala

akka-stream-tests/src/test/scala/akka/stream/impl/fusing/InterpreterSupervisionSpec.scala

@@ -324,7 +324,7 @@ class InterpreterSupervisionSpec extends InterpreterSpecKit {
       val pf: PartialFunction[Int, Int] =
         { case x: Int ⇒ if (x == 0) throw TE else x }
       new TestSetup(Seq(
-        Collect(restartingDecider)(pf))) {
+        Collect(pf, restartingDecider))) {
         downstream.requestOne()
         lastEvents() should be(Set(RequestOne))
         upstream.onNext(2)