usage message

[Henry Cejtin]

It would be nice if the default setting of the option was indicated.  For the
true/false, just listing the default first, perhaps in parens  or  something,
should  be fine.  For the others, it takes more space, but if they can fit it
would be nice.

As to exposing the C back end, how easy is it  to  add  `primitives'  to  the
assembler  back  end?  I suppose that there could be people out there who use
that.

The only serious disadvantage with not being able to restart a
    mlton -stop g ...
is if some one is  actually  modifying  the  assembler  code  generated  (for
instance  by  adding/changing some primitive or something).  It is a bit of a
shame to not do it, but I don't see it as a major problem.