[MLton] Exporting closures to C code

[Matthew Fluet]

> Most C libraries which use callback functions provide some means to
> pass a user-supplied pointer to the callback function.  This could be
> used to invoke MLton functions from C code (passing the closure
> explicitly), even if these functions are not exported.
>
> How hard would that be to implement?

Trivially:

(* An  int -> int  function to export to C. *)
val () = _export "f_callback": int -> int; f

(* Import the address of the exported ML function. *)
val f_addr = _address "f_callback" : MLton.Pointer.t

(* A C function to register an  int -> int  callback. *)
val reg_callback = _import "register_callback": MLton.Pointer.t -> unit;

(* Pass the address of the ML function to the registration function. *)
val () = reg_callback f_addr


This has the very minor disadvantage of introducing "f_callback" as a 
public exported C symbol, rather than keeping it entirely private to the 
ML code.

Also, this uses the SVN HEAD syntax, which isn't exactly the same as that 
available in the latest public release.  However, we should be making a 
new public release "real soon now".