[MLton] Non-exhaustive exn fn match

[Matthew Fluet]

> > 1) Add an "allowFFI {false|true}" annotation, which would simultaneously 
> >    affect "allowImport", "allowExport", and "allowSymbol" (corresponding 
> >    to the new _symbol primitive). 
> 
> Sounds good.  I would also have allowFFI enable _address.

Oh, certainly.  I couldn't remember whether we finally decided to 
introduce 
   _address "symbol" : ptrTy;
or just expect people to use
   #1 (_symbol "symbol" : ptrTy, cbTy;)

> As to the fine grained annotations, they may have looked good at the
> time, but as we add more FFI extensions, I don't really see the point.
> benefit of them.  I'd be just as happy to have only allowFFI, to
> deprecate allow{Export,Import}, and to not add allow{Address,Symbol}.

I think that is reasonable.