CHANGES:

• cohttp-lwt: Do not leak exceptions to Lwt.async_exception_hook. (mefyl #992, #995)
• http.header, cohttp, cohttp-eio: remove "first" and "move_to_first" and the special treatment of the "host" header (mseri #988, #986)
• http.header: introduce "iter_ord" to guarantee iteration following the order of the entries in the headers (mseri #986)
• do not omit mandatory null Content-Length headers (mefyl #985)
• cohttp-async, cohttp-curl-async: compatibility with core/async v0.16.0 (mseri, dkalinichenko-js #976)
• cohttp-lwt server: call conn_closed before drainig the body of response on error (pirbo #982)
• cohttp-eio: Relax socket interface requirement on Server.connection_handler. (mefyl #983)