[MLton] adding synchronous finalizers

[Henry Cejtin]

I  guess that it makes sense that one would still want to have finalizers (or
signals) only done when checking but  still  have  multiple  threads,  so  it
doesn't  sound  like  it  should  be  an  error if a signal handler returns a
different thread.

As to finalizers vs. signals  being  done  differently  (one  automatic,  one
polled),  that  really does look like setting some ref to what should be done
automatically makes sense.