[MLton] ffi improvements

[Matthew Fluet]

> > Oh, BTW is there any reason not to adopt the convention of putting all
> > unsafe interfaces into an Unsafe structure. i.e.
> >
> > MLton.Unsafe.Pointer rather than just MLton.Pointer
> >
> > Just to make sure the user knows what to expect. :)
>
> Good point.  I don't think there are any other unsafe modules in the
> MLton structure -- does anyone else see any?  Given this, I wonder
> whether it is worth doing the move now, or waiting until after the
> release at which point this stuff will probably move to a library.

I vote for after.  MLton.Pointer.* is only as "unsafe" as any of the other
ffi functionality.  I think of it differently than unsafeSub.