Provide interface primitives for the underlying OS file system.

signature OS_FILE_SYS =
sig
  datatype access_mode = A_READ | A_WRITE | A_EXEC
  type dirstream
  eqtype file_id
  val access : string * access_mode list -> bool
  val chDir : string -> unit
  val closeDir : dirstream -> unit
  val compare : file_id * file_id -> order
  val fileId : string -> file_id
  val fileSize : string -> Position.int
  val fullPath : string -> string
  val getDir : unit -> string
  val hash : file_id -> word
  val isDir : string -> bool
  val isLink : string -> bool
  val mkDir : string -> unit
  val modTime : string -> Time.time
  val openDir : string -> dirstream
  val readDir : dirstream -> string option
  val readLink : string -> string
  val realPath : string -> string
  val remove : string -> unit
  val rename : {old : string, new : string} -> unit
  val rewindDir : dirstream -> unit
  val rmDir : string -> unit
  val setTime : string * Time.time option -> unit
  val tmpName : unit -> string
end