[MLton] filedes = int (was: Stack size?)

[Stephen Weeks]

> Now that there is a complete front-end, we could acheive the same by
> 
> -	datatype file_desc = FD of int
> +	structure FD :> sig type t end = struct type file_desc = int end
> +	type file_desc = FD.t

I like this solution, and it is used in other places in the basis
library.  I'm tempted to make the change.

> Though I think there is still some advantage (though I'm hard
> pressed to put my finger on exactly what the advantage is) to
> requiring explicit pack and unpack functions to witness the
> coercion.

This would only be at the FFI, right?