spray-json, akka-http, slick shouldn't depend on instances

[luksow]

spray-json, akka-http, slick modules depend on instances which is, in general, unnecessary, even though most users would probably add instances as a dep.

I think the correct solution would be to push InstanceConverter trait to macro-utils to sit there together with CaseClass1Rep or even better, create a thin module core or base and have InstanceConverter and CaseClass1Rep there.

[luksow]

In case https://github.com/theiterators/kebs/pull/214 is merged, play-json & circe should be fixed as well.

[pk044]

Done in https://github.com/theiterators/kebs/pull/269