+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