Cohttp 0.18.0 compatibility

[rgrinberg]

Use Request/Response directly from Cohttp base module. The lwt specific
versions have been removed in cohttp 0.18.0 since they were the same
anyway.

I think this should make ketrew compatible with 0.17.0 and 0.18.0

[smondet]

I did something similar a few minutes ago :)

805e6b7

I used Cohttp_lwt_unix in case the types change in the future
https://github.com/mirage/ocaml-cohttp/blob/master/lwt/cohttp_lwt.mli#L40
(e.g. for Body.t they are different for a good reason:
https://github.com/mirage/ocaml-cohttp/blob/master/lwt/cohttp_lwt.mli#L40).

[rgrinberg]

They are different for a good reason but that's another difference we're trying to get rid of :)

We're trying to get rid of the backend specific request/response modules because all they do is expose unnecessary implementation details. Just a warning not to rely on the backend specific bits request/response because they're mostly there to implement the cohttp server/client in lwt/async.

This is why I recommend just to use Cohttp.{Request,Response}

[smondet]

OK, I'll merge yours :)

BTW for Body.t, the special case of a “stream” is easy to abstract; a stream can just be a unit -> string option IO.t function.

[rgrinberg]

Yeah i have something like that in the works for body. The demand for it has proven to be pretty low though because streams are discouraged in async anyway.