=str - Various Flow and FlowMaterializer improvements

THIS IS A WIP-BRANCH/PR for discussion and improvement, DO NOT MERGE YET


  - 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`
> You should go with IteratorPublisher then. The IterableSource can always create a new Iterator from the Iterable on attach anyway.

I'm not sure I understand what to do though—should I keep any parts of IterableSource?