[MLton] unused variables in MLton

[Matthew Fluet]

> I've finished going through MLton and eliminating unused variables.
> All that remain are either in the automatically generated code for the
> front end or in the x86-codegen.  Matthew, I'd rather you look at the
> latter, since you are more likely to detect if one indicates a bug.

Sure thing; probably over the weekend.

> Also, how are you coming along on the sequence-unit stuff?  I'd be
> happy to work on that.

I'm done with /lib/mlton, /mlton (except /mlton/elaborate); I'll check
those in later this evening (they are on my home machine and I need to
check that there aren't conflicts with the unused variables changes).
I haven't looked at /mlyacc, /mllex, /mlprof.