waits for it to complete before returning.
<item> <tt/mkWeak/ now takes a <tt/Maybe (IO ())/ for the finalizer,
and <tt/mkWeakNoFinalizer/ is removed.
waits for it to complete before returning.
<item> <tt/mkWeak/ now takes a <tt/Maybe (IO ())/ for the finalizer,
and <tt/mkWeakNoFinalizer/ is removed.