Cohttp.0.13.0

Compatibility breaking interface changes:

* Add sexp converters for Conduit contexts and `Lwt` client and server
  modules and module types.

New Features and bug fixes:
* Can use the Conduit 0.7+ `CONDUIT_TLS=native` environment variable to
  make HTTPS requests using the pure OCaml TLS stack instead of depending
  on OpenSSL bindings.  All of the installed binaries (client and server)
  can work in this mode.
* Add `Cohttp_lwt_unix_debug` which lets libraries control the debugging
  output from Cohttp.  Previously the only way to do this was to set the
  `COHTTP_DEBUG` environment variable at the program start.
* Add `cohttp-curl-lwt` as a lightweight URI fetcher from the command-line.
  It uses the `cmdliner` as a new dependency.
* Remove build dependency check on `lwt.ssl` for `cohttp.lwt`.
  This has been moved to conduit, so only `lwt.unix` is needed here now.