VM: each_object() can't iterate the nursery so instead we assume it's empty
VM: get rid of the duplicated nursery instance in vm->data->nursery

vm.nursery and vm->data->nursery are different objects. They get out
of sync when Factor code bumps vm.nursery but leaves vm->data->nursery
unchanged. The emptiness of vm->data->nursery meant that each_object()
never iterated it.
VM: move next_object_after() and first_object() to aging_space (no iteration possible in nursery)