0.8.21.31: tweak finalizers, thighten spec further

0.8.21.31: tweak finalizers, thighten spec further
 * sprinkle WITHOUT-GCING around, so that we won't enter GC while
    holding the lock on finalizer store.
 * specify that finalizers run in an unpredictable dynamic scope and
    must be fully re-entrant. Add a few examples for good measure.
 * add finalizer, weak pointer, and after gc hook documentation to the
    manual.