=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-http-tests/src/test/scala/akka/http/server/directives/CodingDirectivesSpec.scala

@@ -158,7 +158,7 @@ class CodingDirectivesSpec extends RoutingSpec {
     "correctly encode the chunk stream produced by a chunked response" in {
       val text = "This is a somewhat lengthy text that is being chunked by the autochunk directive!"
       val textChunks =
-        text.grouped(8).map { chars ⇒
+        () ⇒ text.grouped(8).map { chars ⇒
           Chunk(chars.mkString): ChunkStreamPart
         }
       val chunkedTextEntity = HttpEntity.Chunked(MediaTypes.`text/plain`, Source(textChunks))