[MLton] sequenceUnit vs warnSequenceUnit

[Matthew Fluet]

> > I think the unusedIdentifier will be a little trickier, since its
> > semantics are not as local as the others.
> 
> It will be a little trickier, but not much, I hope, because in this
> case the semantics is local -- what matters is the value of the
> unusedIdentitier annotation when the identifier is defined.  In
> elaborate-env.fun, NameSpace.newUses can record the value of the
> annotation, and then processDefUse can propagate the value to the
> point of the error/warning message.

Fair enough.