Remove servant-server dependency in core

[domenkozar]

This allows packages with only api combinators not to depend on servant-server.

I have WIP branch dropping some ancient servant versions compatibility, but it would need also to move swaggerSchemaUIServerImpl out of core.

What do you think @phadej

[phadej]

where swaggerSchemaUIServerImpl will go then?

[domenkozar]

It would have to be a separate package probably, maybe best to wait on multiple libraries support in Cabal.

[phadej]

Ok. Let's drop some older servant's when the next one is out. (I think I'll finish the Stream refactor, pick low hanging fruits and make 0.15 during October).