[MLton] Callback to function pointer?

[Stephen Weeks]

>  _address "symbol";		==> MLton.Pointer.t
>  _store *: 'a;		==> (MLton.Pointer.t * 'a) -> unit
>  _store "symbol": 'a;		==> 'a -> unit
>  _import *: 'a;		==> MLton.Pointer.t -> 'a
>  _import "symbol": 'a;	==> 'a
>  _export "symbol": 'a;	==> 'a -> unit

I like this.  I agree with others that it would be useful to have
_address specify a type, which must expand to MLton.Pointer.t, so that
one can import the addresses at opaque types.

One other nice thing about the above is that it is, I believe,
strictly an extension of our currently documented FFI.