Faraday_asyncval serialize :
Faraday.t ->
yield:(Faraday.t -> unit Async.Deferred.t) ->
writev:
(Faraday.bigstring Faraday.iovec list ->
[ `Ok of int | `Closed ] Async.Deferred.t) ->
unit Async.Deferred.tval writev_of_fd :
Async.Fd.t ->
Faraday.bigstring Faraday.iovec list ->
[ `Ok of int | `Closed ] Async.Deferred.t