!str #16902: Unify stream internal representation

also =str #16912: Fix StreamTcpSpec flakiness

akka-stream/src/main/scala/akka/stream/Graph.scala

@@ -0,0 +1,23 @@
+/**
+ * Copyright (C) 2015 Typesafe Inc. <http://www.typesafe.com>
+ */
+package akka.stream
+
+import akka.stream.impl.StreamLayout
+
+trait Graph[+S <: Shape, +M] {
+  /**
+   * Type-level accessor for the shape parameter of this graph.
+   */
+  type Shape = S
+  /**
+   * The shape of a graph is all that is externally visible: its inlets and outlets.
+   */
+  def shape: S
+  /**
+   * INTERNAL API.
+   *
+   * Every materializable element must be backed by a stream layout module
+   */
+  private[stream] def module: StreamLayout.Module
+}