The most important changes from spray-routing:
- Change Route from `RequestContext => Unit` to `RequestContext => Deferrable[RouteResult]`
- Switch directive extraction model from shapeless HLists to plain Scala Tuples
- Switch API from promoting actors as Route "containers" to being powered by plain executors/flow-materializers
- Make `Directive0` re-execute inner routes per requests, refs spray/spray#780
Other fixes:
- Simplify unmarshalling infrastructure, now based on `Deferrable`, closes#15690
- Change marshalling infrastrucutre to be based on `Deferrable` as well
- Clean up `akka.http` package contents, closes#15689
Changing from HList-based extractions to Tuple-based ones comes with one important drawback:
We can't rely on type bounds anymore to tighten and auto-document the API where generic extractions handled.
Currently we therefore fall back to context bounds where they add value, but this might change if the added overhead or code complexity becomes too much. We'll see when the remaining parts of spray-routing are ported.
