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/operators/Map.scala

@@ -14,6 +14,6 @@ object Map {
 
   //#map
   val source: Source[Int, NotUsed] = Source(1 to 10)
-  val mapped: Source[String, NotUsed] = source.map(elem ⇒ elem.toString)
+  val mapped: Source[String, NotUsed] = source.map(elem => elem.toString)
   //#map
 }

akka-docs/src/test/scala/docs/stream/operators/SourceOperators.scala

@@ -23,7 +23,7 @@ object SourceOperators {
     implicit val materializer: ActorMaterializer = ActorMaterializer()
 
     val source: Source[Int, NotUsed] = Source.fromFuture(Future.successful(10))
-    val sink: Sink[Int, Future[Done]] = Sink.foreach((i: Int) ⇒ println(i))
+    val sink: Sink[Int, Future[Done]] = Sink.foreach((i: Int) => println(i))
 
     val done: Future[Done] = source.runWith(sink) //10
     //#sourceFromFuture

akka-docs/src/test/scala/docs/stream/operators/SourceOrFlow.scala

@@ -30,9 +30,9 @@ object SourceOrFlow {
     //#conflate
     import scala.concurrent.duration._
 
-    Source.cycle(() ⇒ List(1, 10, 100, 1000).iterator)
+    Source.cycle(() => List(1, 10, 100, 1000).iterator)
       .throttle(10, per = 1.second) // faster upstream
-      .conflate((acc, el) ⇒ acc + el) // acc: Int, el: Int
+      .conflate((acc, el) => acc + el) // acc: Int, el: Int
       .throttle(1, per = 1.second) // slow downstream
     //#conflate
   }
@@ -45,9 +45,9 @@ object SourceOrFlow {
       def sum(other: Summed) = Summed(this.i + other.i)
     }
 
-    Source.cycle(() ⇒ List(1, 10, 100, 1000).iterator)
+    Source.cycle(() => List(1, 10, 100, 1000).iterator)
       .throttle(10, per = 1.second) // faster upstream
-      .conflateWithSeed(el ⇒ Summed(el))((acc, el) ⇒ acc sum Summed(el)) // (Summed, Int) => Summed
+      .conflateWithSeed(el => Summed(el))((acc, el) => acc sum Summed(el)) // (Summed, Int) => Summed
       .throttle(1, per = 1.second) // slow downstream
     //#conflateWithSeed
   }
@@ -61,7 +61,7 @@ object SourceOrFlow {
 
     //#scan
     val source = Source(1 to 5)
-    source.scan(0)((acc, x) ⇒ acc + x).runForeach(println)
+    source.scan(0)((acc, x) => acc + x).runForeach(println)
     // 0  (= 0)
     // 1  (= 0 + 1)
     // 3  (= 0 + 1 + 2)