[MLton] more FFI revisions

[Matthew Fluet]

> > I can also see the argument that for an _extension_, a syntax that
> > isn't easily mistaken for ML is a win.
> 
> We've already got "_" before the keyword to do that.  I think the
> analogy with the usual use of ":" is pretty strong.  In fact, that's
> what we started with originally with _ffi (which became _import).
> It's just that we got clever to save a few keystrokes ("-> unit") when
> we added _export.

Alright; and there were a lot of previous pushes for using the ":" as a
true annotation.