=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`
I don't think it is releasable with `SynchronousPublisherFromIterable`, unless you can convince us that it is safe, i.e. I would not like to merge this until we have sorted this out.

Can't we use IteratorPublisher?

BTW, IteratorPublisher should be simplified by removing the multi-subscriber support. I'll create another ticket for that.