+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
+htp resurrect RequestContext.flowMaterializer
+htc a safer Future.awaitResult that doesn't throw away the call stack of the awaiter
=htp #16238 fix RangeDirectives not to assume reusable entity data stream
+str add akka.stream.materializer.file-io-dispatcher setting
+htp #15922 import FileAndResourceDirectives from spray
!htc,par cleanup warnings

 - about Rendering not being accessible for implementors
 - about DepFn1/2 being defined in package object
=pro fix the parallelExecution setting broken in #16185