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

@@ -37,9 +37,9 @@ class TakeLastSinkSpec extends StreamSpec {
 
       val result: Future[Seq[Student]] = sourceOfStudents.runWith(Sink.takeLast(3))
 
-      result.foreach { topThree ⇒
+      result.foreach { topThree =>
         println("#### Top students ####")
-        topThree.reverse foreach { s ⇒
+        topThree.reverse foreach { s =>
           println(s"Name: ${s.name}, GPA: ${s.gpa}")
         }
       }