[MLton] mlb support

[Stephen Weeks]

> 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.

Maybe.  Although it's lots of flags, it does seem nice to be able to
have a master control on the command line.  Perhaps we could use two
flags with lots of options?

	-enable-ann {warnMatch|warnUnunsed|sequenceUnit}
	-disable-ann {warnMatch|warnUnunsed|sequenceUnit}

> > 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.

"all of type checking except for match compilation"

If that's accurate, perhaps a better name would be "tc-no-match".