[MLton] sequenceUnit vs warnSequenceUnit

[Stephen Weeks]

> 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.