[MLton] mlb support

[Stephen Weeks]

> What do people think that the behavior of
> 
>   -disable-ann sequenceUnit -sequence-unit true
> 
> should be?  I see two options:
>  1) -disable-ann means that the annotation is ignored in .mlb files;
>     it is as if it weren't there.
...
> Currently behavior 2 is implemented, but I think behavior 1 is
> preferable.

I prefer (1) as well.

Certainly, the interactions between the following pairs of options
should be consistent.

	-sequence-unit 	-disable-ann sequenceUnit
	-warn-match	-disable-ann warnMatch
	-warn-unused	-disable-ann warnUnused

So, if

	-disable-ann sequenceUnit -sequenceUnit true

causes the sequenceUnit requirement to be enforced, then so should

	-disable-ann warnUnused -warn-unused true

cause warnings to be displayed (it currently does not).  And so should

	-disable-ann warnMatch -warn-match true

cause warnings to be displayed (it currently does not).