[MLton] sequenceUnit vs warnSequenceUnit

[Matthew Fluet]

> > How about the following?
> > 
> >   nonexhaustiveMatch {warn|error|ignore}
> >   nonexhaustiveExnMatch {warn|error|ignore}
> >   redundantMatch {warn|error|ignore}
> >   sequenceNonUnit {ignore|error|warn}
> >   unusedIdentifier {ignore|error|warn}
> 
> I like this. If you like, I could try to produce a patch for this. Can
> you see any major complications?

With the exception of unusedIdentifier, I don't think it will be all that 
difficult.  I think the unusedIdentifier will be a little trickier, since 
its semantics are not as local as the others.