=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
=str #16321 Harden FlexiMergeTest
=str #16318 Harden FlexiRouteTest
+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
Merge pull request #16362 from akka/wip-16318-FlexiRouteTest-patriknw

=str #16318 Harden FlexiRouteTest
Merge pull request #16360 from akka/wip-16321-FlexiMergeTest-patriknw

=str #16321 Harden FlexiMergeTest
=pro update to reactive-streams 1.0.0.M1
Notice that one 317 is disabled and awaiting a fix in the TCK
Merge pull request #16384 from ktoso/rs-100M1-ktoso

=pro update to reactive-streams 1.0.0.M1
=htc #16388 fix StackOverflowException during chunk parsing
Merge pull request #16390 from spray/w/16388-fix-SOE-in-chunk-parsing

=htc #16388 fix StackOverflowException during chunk parsing
-str #16393 removes timerTransform from public API
=htc fix broken per-connection header cache