replace unicode arrows

* ⇒, →, ←
* because we don't want to show them in documentation snippets and
  then it's complicated to avoid that when snippets are
  located in src/test/scala in individual modules
* dont replace object `→` in FSM.scala and PersistentFSM.scala

akka-docs/src/test/scala/docs/stream/GraphCyclesSpec.scala

@@ -13,7 +13,7 @@ class GraphCyclesSpec extends AkkaSpec {
   implicit val materializer = ActorMaterializer()
 
   "Cycle demonstration" must {
-    val source = Source.fromIterator(() ⇒ Iterator.from(0))
+    val source = Source.fromIterator(() => Iterator.from(0))
 
     "include a deadlocked cycle" in {