remove temporarily dead code
Move RouteResult ADT types into RouteResult companion
Disable RouteResult creation by user
= htp: larger refactoring

 Mainly:
 - introduce `Deferrable` abstraction, now used for RouteResult and marshalling
 - move ExecutionContext, FlowMaterializer and request log into RequestContext
 - (temporarily) remove code that is not directly used yet

  akka-http-testkit and akka-http-tests not currently compiling!