=str - Various Flow and FlowMaterializer improvements

- Switches from using size-of-1/2 Vectors to using Lists
  - Fixes an issue where processorForNode wouldn't use the dispatcher form the settings
  - Adds a dedicated Collect fusion op
  - Adds various simplifications to ActorBasedFlowMaterializer
  - Adds FIXMEs where appropriate
  - Switches `grouped` to use a VectorBuilder
  - Adds support for `scan`
  - ActorBasedFlowMaterializer now uses Iterator instead of head+tail decomp on Seqs
  - Identity and Completed Transformers are now cached
  - Adds dedicated AstNodes for all combinators
  - Adds a hook-in point for fusion in `ActorBasedFlowMaterializer`
  - Adds support for `Operate` an AstNode with a function that create a fusing.Op
  - Adds experimental and slow optimizer as a PoC
  - Adds verification that Ast.Fused does not exist in optimizer input

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

@@ -6,10 +6,7 @@ package akka.stream
 import java.util.Locale
 import java.util.concurrent.TimeUnit
 
-import akka.stream.impl.ActorBasedFlowMaterializer
-import akka.stream.impl.Ast
-import akka.stream.impl.FlowNameCounter
-import akka.stream.impl.StreamSupervisor
+import akka.stream.impl._
 
 import scala.collection.immutable
 
@@ -63,7 +60,8 @@ object FlowMaterializer {
       materializerSettings,
       context.actorOf(StreamSupervisor.props(materializerSettings).withDispatcher(materializerSettings.dispatcher)),
       FlowNameCounter(system).counter,
-      namePrefix)
+      namePrefix,
+      optimizations = Optimizations.none)
   }
 
   /**