[MLton] mlb support

[Stephen Weeks]

> > 	-enable-ann {warnMatch|warnUnunsed|sequenceUnit}
> > 	-disable-ann {warnMatch|warnUnunsed|sequenceUnit}
> 
> Why would we need both -enable and -disable?

I added -enable-ann by analogy with how we handle bool args,
e.g. -safe {false,true}, where it seems cleaner to have a way to
specify both options with a flag, rather than only being able to
change the default.

> By -enable, you don't mean to force the option, overriding an
> annotation?

No, merely that the annotations are enabled (i.e. the default).

I'm not convinced in this case that it's worth having -enable-ann,
although neither would I be surprised to find us wanting it later.