Clean up signatures - First attempt

My first attempt to address issue #110. First by fixing the backend agnostic parts.

* I've moved all signatures to `Cohttp.S`. Happy to hear suggestions for a better name.

* I did not move `IO.S` to `Cohttp.S.IO` because some users could still depend on it. I'm happy to do it though if I get the go ahead.

* Removed the `Cohttp.Client` because it doesn't seem like anybody is using it.

As trivial as this stuff looks it took me a bit longer than I expected :/ This is why I'm choosing to submit this halfway without Lwt which I think is where the big clean up lies.