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)

Btw. the reason is that it is costly to provide continuations from all possible intermediate states. This way parsing usually continues until there's a data underrun. When new data arrives parsing is continued from the last "snapshot position" where a continuation was provided, appending new data to unparsed data from the last run.