Per-task nurseries for SMP.  This was kind-of implemented before, but
it's much cleaner now.  There is now one *step* per capability, so we
have somewhere to hang the block count.  So for SMP, there are simply
multiple instances of generation 0 step 0.  The rNursery entry in the
register table now points to the step rather than the head block of
the nurersy.