[MLton] mlb support

[Matthew Fluet]

> > > 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".

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.