=str #16085 PublisherSink is no longer a singleton

* rename existing Source/Sink tests to the latest names
* uncomment TickSource test with Zip
+htp #15924 import FutureDirectives from spray
Merge pull request #16091 from spray/w/15924-future-directives

+htp #15924 import FutureDirectives from spray
=htc smaller improvements in FastFuture
=htp add support infrastructure for folding over tuples with a binary poly-function
=htp enable tests that were erroneously disabled due to specs2 -> scalatest copy-pasta
=htp small improvement to Marshaller
!htp #15935 clean up and extend unmarshalling infrastructure
=htp refactor tuple join to foldLeft-based implementation, unify tuple operations
+htp #15929 port ParameterDirectives from spray
!htp #16101 improve basic directive infrastructure, add more scaladoc, fix smaller problems
Merge pull request #16094 from spray/wip-15929-mathias

Import ParameterDirectives from spray
+htp #15931 Import RespondWithDirectives from spray
+htp #15918 import CacheConditions from spray
+htp #15919 import CookieDirectives
!htp rename conflicting withExecutionContext directive to extractExecutionContext
+htp add RequestContext/BasicDirectives.withLog/extractLog
Merge pull request #16108 from spray/w/port-more-directives

Fixes for 15931, 15918, 15919
Merge pull request #16111 from 2m/wip-unsingle-publisher-drain

=str #16085 PublisherSink is no longer a singleton
Merge pull request #16107 from ktoso/wip-cleanup-javadsl-ktoso

=str #16103 Flattening FlowOps into Flow (same for Sink / Source)