Compression support for cohttp-lwt client using decompress

The library interface tries to stay somewhat close to dream-encoding.

Usage example:

let get_with_compression_support ?headers uri =
  let headers = Clz_cohttp.update_headers headers in
  let open Lwt.Syntax in
  let* resp, body = Cohttp_lwt_unix.Client.get ~headers uri in
  let status = Cohttp_lwt.Response.status resp in
  let* () = if status <> `OK then Cohttp_lwt.Body.drain_body body else Lwt.return_unit in
  match status with
  | `OK ->
    let body = Clz_cohttp.decompress (resp, body) in
    Lwt.return body
  | _ -> Lwt.fail_with "Not Ok"