Split up HasServer, HasClient

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)