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-actor-tests/src/test/scala/akka/pattern/PatternSpec.scala

@@ -15,7 +15,7 @@ object PatternSpec {
   final case class Work(duration: Duration)
   class TargetActor extends Actor {
     def receive = {
-      case (testLatch: TestLatch, duration: FiniteDuration) ⇒
+      case (testLatch: TestLatch, duration: FiniteDuration) =>
         Await.ready(testLatch, duration)
     }
   }