[MLton] more FFI revisions

[Henry Cejtin]

For the FFI thing (which I haven't had time to really follow yet), please,
please, don't use comma-separated types (for "symbol" and *).  Yes a single
type which must be a 2-tuple.  It makes way more sense since that is atleast
something that already exists in ML.  Also it is something that you can name
(via a type abbreviation) which can be useful.

Actually, I would way way prefer if you just specified the full type you get,
even though it involves some duplication.  After all, if that is painful then
I can (or should) be able to use a type polymorphic type abbreviation for that.
I.e.,
	type ('ptrTy, 'cbTy) t = ('prTy -> 'cbTy) * ('prTy * 'cbTy -> unit)
and then I use
	_symbol * (???, ???) t;
This strikes me as WAY WAY better.