Provide interface primitives for the underlying OS IO.

signature OS_IO =
sig
  eqtype iodesc
  eqtype iodesc_kind
  eqtype poll_desc
  type poll_info
  exception Poll
  structure Kind : sig
    val device : iodesc_kind
    val dir : iodesc_kind
    val file : iodesc_kind
    val pipe : iodesc_kind
    val socket : iodesc_kind
    val symlink : iodesc_kind
    val tty : iodesc_kind
  end
  val compare : iodesc * iodesc -> order
  val hash : iodesc -> word
  val infoToPollDesc : poll_info -> poll_desc
  val isIn : poll_info -> bool
  val isOut : poll_info -> bool
  val isPri : poll_info -> bool
  val kind : iodesc -> iodesc_kind
  val poll : poll_desc list * Time.time option -> poll_info list
  val pollDesc : iodesc -> poll_desc option
  val pollIn : poll_desc -> poll_desc
  val pollOut : poll_desc -> poll_desc
  val pollPri : poll_desc -> poll_desc
  val pollToIODesc : poll_desc -> iodesc
end