We were assuming that falling_nodes would settle in an
orderly fashion and just calling the node placement code, but
not the displacement of old nodes.
Make sure that things aren't destroyed in the landing zone.
Before, settling was being processed out of order, so nodes could
fall into the space of nodes below and kick them out as items, so a
solid column of nodes could collapse and eject some nodes as items.
This makes hopper-feeding systems a pain to use.
Instead, explicitly signal upwards (tail-recursive) to settle all
falling nodes on the same tick, since they should already be falling
and accelerating downwards in lock-step.
This isn't perfect, as there are possible race conditions with things
being added into the node-space of the falling column while it's
falling, but it seems to work for now...
