CoHTTP implementation using the Lwt concurrency library

This is a portable implementation of HTTP that uses the Lwt concurrency library to multiplex IO. It implements as much of the logic in an OS-independent way as possible, so that more specialised modules can be tailored for different targets. For example, you can install cohttp-lwt-unix or cohttp-lwt-jsoo for a Unix or JavaScript backend, or cohttp-mirage for the MirageOS unikernel version of the library. All of these implementations share the same IO logic from this module.