[MLton] Windows ports and paths

[Stephen Weeks]

> The obvious correct implementation is
> 
>         fun joinDirFile {dir, file} =
>             let
>                 val {isAbs, vol, arcs} = fromString dir
>             in
>                 toString {isAbs=isAbs, vol=vol, arcs=arcs@[file]}
>             end

Both this suggestion and the previous one mean that

  OS.Path.joinDirFile {dir = "b/", file = "c"}

evaluates to "b//c", unlike "b/c" which is what every SML
implementation says.

I think it makes more sense to remove a trailing empty arc using
concatArcs, just as concat does.  So, I propose

fun joinDirFile {dir, file} =
   if dir = "" andalso file = ""
      then ""
   else
      let
	 val {arcs, isAbs, vol} = fromString dir
      in
	 toString {arcs = concatArcs (arcs, [file]),
		   isAbs = isAbs,
		   vol = vol}
      end

I needed the special case for both dir and file being empty because
otherwise toString raises Path.