akka-http-core for merge

This is the latest state of akka-http-core:

- Everything wired up properly against current akka-stream
- All tests green
- `test:run` gives you an HTTP server on port 8080 to play with

After merge these things are still missing for M1:

- Merge of Java API (basically ready but not yet part of this PR)
- client-side request rendering / response parsing (in progress)
- docs (in progress)

For me the current way parses like (while skimming) "Protected[blahblah].enable" which kind of implies that I enabled some form of protection, while it is actually about removing that protection. So it is about the ambiguity between "enabling the creation of protected headers" vs "enable the protection against creating certain headers". Maybe "AllowProtectedHeaderCreation.enable"? Or something like that.