[MLton] mlb support

[Stephen Weeks]

> > If that's accurate, perhaps a better name would be "tc-no-match".
> 
> I think I'm favoring this option.  Note that with implicitly wrapping a
> .sml file as an mlb expression, to get the elaborateOnly behavior, one
> would need to compile with -disable-ann warnMatch, which seems kind of odd
> when compiling a .sml file.

It doesn't bother me, but I don't know yet how explicit the user guide
is going to be on how .sml files are implictly wrapped as .mlb files.

In any case, either way (-stop tc-no-match or -disable-ann warnMatch)
is fine with me.  -disable-ann has the benefit of being able to
disable warning messages while doing a full compile, which seems
nice.