=str - renames ReactiveStreamsConstants to ReactiveStreamsCompliance
Merge pull request #16333 from akka/wip-rename-reactivestreamsconstants-√

=str - renames ReactiveStreamsConstants to ReactiveStreamsCompliance
=str #16338 Emit final elements in mapConcat
Merge pull request #16340 from akka/wip-16338-mapConcat-patriknw

=str #16338 Emit final elements in mapConcat
=str #16349 fix map + map optimization
Merge pull request #16349 from 2m/wip-16349-fix-map-map-optimization

=str #16349 fix map + map optimization
!str #15236 Replace Transformer with Stage

* replace all existing Transformer with Stage (PushPullStage)
* use Flow[ByteString, ByteString] as encoder/decoder transformer in http
* use the IteratorInterpreter for strict if possible
* emit then become
* emit then finish
* termination emits
* FlowTransformerSpec
* rework types to work with Java API
* rename and move things
* add scaladoc
Merge pull request #16287 from akka/wip-15236-transformer-patriknw

!str #15236 new transformer
+htc apply smaller extensions and simplifications
!htp #16190 refactor and improve marshalling infrastructure, get rid of `Marshallers` abstraction altogether

Also improve Unmarshalling infrastructure once more.
!htk rename `RouteTest::entity` to `RouteTest::responseEntity`, smaller test cleanups

Also upgrade to latest (un)marshalling changes.
!htp #15927 port `MarshallingDirectives` from spray
!htp #15923 port `FormFieldDirectives` from spray
Merge pull request #16292 from spray/wip-15923-mathias

MarshallingDirectives, FormFieldDirectives, and more
=str #15152 ast transformations

* for feedback
* supports all linear AST nodes