=htp groundwork for routing and a first, basic set of directives

This is a first cut extracted from #15609 ready to be reviewed.

It contains 
 - The basic data structures for routing
 - `BasicDirectives` containing helper directives used for building other directives
 - `PathDirectives` + `MethodDirectives`, so that one can build simple demos and as a demonstration of how directives are built
 - A first implementation of `HttpService` on top of streams
 - a new `akka-http-testkit` project containing our testing infrastructure
 - a new `akka-http-tests` project containing tests for the current state + a simple, standalone test application demonstrating routing on top of streams
 - supporting changes in the existing infrastructure

Even in this form it is quite big and much stuff to review so I'm not sure if it is still viable to be included in the 0.6 milestone.

/cc @sirthias 
Hmm... I see. 
Johannes has a variant of `Deferrable` that is value-class based and doesn't support "mapping compression" on `NonStrict`. I'd like to bench these things in a somewhat real-world API definition to better understand the trade-offs.
Can we agree to keep things as they are for the time being and add respective documentation as well as "TODO" reevaluate lazy mapping after benchmark" marker?