signature golfing the client api

I know this is borderline too cute but the goal is to make the documentation a little more terse and yet still exhaustive. Any name changes/suggestions are all welcome. We also don't have to go all the way and choose a middle ground. I went as far as I could here.

Testing the waters with changes like these. I know I found all these typedefs annoying at first but began to appreciate them. Perhaps we can reuse some of these types with the lwt client?

P.S. `call` could be golfed to `(Cohttp.Status.meth -> full_response) request with_body` if we flip `Uri.t` and `Cohttp.Status.meth` but I don't think it's worth creating churn over stuff like that.