+str - Changes the signature of Source.expand to be curried, to guide type inference.
+str - Fixes #16065 by adding an IteratorPublisher and removing SimpleCallbackPublisher

Adds an IteratorPublisher and IteratorPublisherImpl
Removes the SimpleCallbackPublisher
Switches the callback-source () => Option[T] to use a new ThunkIterator and the IteratorPublisher
Removes the use of a ControlThrowable (Stop) for signalling end since we now use Iterator
Improves a couple of streams-related tests so when they fail we get better output
Merge pull request #16253 from akka/wip-fix-expand-signature-√

+str - Changes the signature of Source.expand to be curried, to guide ty...
Merge pull request #16251 from akka/wip-16065-collapse-thunksource-into-iteratorsource-√

+str - Fixes #16065
=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 experimental and slow optimizer as a PoC
Wip - improves optimizer code organization and makes the optimizer configurable