[MLton] mlb support

[Matthew Fluet]

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

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.  Therefore, every SML file is elaborated
    with the defaults as implied by the command line.  So, in the above,
    everything would be compiled with sequences having unit type.
 2) -disable-ann means that the option is set to the "default" default;
    this overrides other command line options.  So, in the above,
    everything would be compiled with sequences not necessarily having
    type unit, because the default for -sequence-unit is false.

Behavior 2 might be a little confusing.  The argument in favor of behavior
1 is that to get elaborateOnly, one needs to compile with
  -disable-ann warnMatch -warn-match false

Currently behavior 2 is implemented, but I think behavior 1 is
preferable.