Split up HasServer, HasClient

[fisx]

For unit-testing, we need HasClient instance that gives us clientWithRoute, but the instance does not require a RunClient instance.

The easy solution is to write a RunClient instance using undefined, but it would be nicer to keep separate things separate:

type HasClient m api = (RunClient m, HasClient' m api)
class HasClient' m api where ...

... and then require RunClient where necessary in order to implement the HasClient' instance, instead of in the class.

Any concerns or objections? If not I may make a PR.

(ping @pcapriotti)

[fisx]

the down side i guess is that the constraints will say HasClient, and the instances will be of HasClient'. not entirely pretty.

[fisx]

or we could drop HasClient and always add an explicit RunClient where needed, also where the instance is used?