Add a function to cast Lwt_stream to Ocsigen_stream
Add dependency Cohttp.lwt
Add a dummy receiver generator (incompatible with Cohttp but necessary for request_info)
Add different cast between http_frame and Cohttp.Request.t (is experimental !)
Move generation of POST data from Ocsigen_brouette to Ocsigen_generator and add a cast between Cohttp.Request.t to request_info
Breaks the dependency with Ocsigen_extensions