[MLton] mlb support

[Matthew Fluet]

> > On the other hand, there could be situation where you would not see
> > a warning when compiling with -warn-match false -stop tc and would
> > see the warning when compiling with just -warn-match false.
>
> Perhaps we need two flags
>
>   -enable-warn-match {false|true}
> 	Governs whether there can be match warnings at all.
>
>   -warn-match {false|true}
> 	The default warnMatch annotation for mlbs.

If we provide enable for warn-match, then I would want it for other
annotations, like warn-unused and sequence-unit.  Maybe that is o.k.

> Or perhaps we should just add a new -stop, "elab".

That is another option.  Although it is a little tough to describe exactly
what -stop elab does compared to -stop tc.