[MLton] Callback to function pointer?

[Stephen Weeks]

> "&" would probably be better, but I chose "#" because it was already known 
> by the lexer.  Since "&" is a valid SML identifier (and "#" is not), it 
> complicates the lexer/parser (admittedly, not by that much).
> 
> Another alternative is a different keyword:
> 
>   _address "name": MLton.Pointer.t;

How about this, and dropping the ": MLton.Pointer.t;", since that's
the only reasonable type for an address?

   _address "name"