Merge pull request #16089 from spray/w/16088-fix-continuation-handling

16087, 16088 fix RouteResult handling support in routing infrastructure
+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 refactor tuple join to foldLeft-based implementation, unify tuple operations
=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 improve basic directive infrastructure, add more scaladoc, fix smaller problems
+htp #15929 port ParameterDirectives from spray