[MLton] pinned objects (was: implement _address and _symbol)

[Stephen Weeks]

> So, to make sure I understand, a pin is a pointer into the heap from 
> outside of it. When a GC occures, the external pointer gets updated.

Yes.

> The way I see it, Pinned.t is (yet another) level of pointers. Here,
> the Pinned.t lives on the heap, but points at the pin which lives
> outside the heap, which points to the 'a which lives in the heap.

No extra level.  Underneath the :> we would have
  
  type MLton.Pinned.t = MLton.Pointer.t

There is no need for another level beyond the pin that lives outside
the heap.  The ML value is the pin (address of the malloc'd object).

> This works perfectly with what I now know of the FFI, since Pinned.t
> would be internally exactly the pointer you want to pass to the C
> function.

Right.