!str #15672 shallow shim of the new Streams DSL

* mostly Flow API
* initial Graph API
* positive and negative compilation tests
!str #15672 added all combinators from current Flow API

* combinator tests
!str #15672 add HTTP Pipeline with the new DSL

* implement graph operations as case class junctions
* add materialization method to ClosedFlow
!str #15672 add a way to retrieve Publsiher or Subscriber from Output/Input

* also remove ClosedFlow.as(...)
Merge pull request #15698 from 2m/wip-typesafe-dsl

WIP Steams typesafe DSL