[MLton] ffi improvements

[Stephen Weeks]

> 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.