There is now an inclusive_descendants_version field of each node, which
increases each time the node, or any of its descendants, is dirtied.
This can be used for cache invalidation, by caching a version number
and comparting the current version number against the cached version number.
After rebasing, this suddenly became a problem again, even though there's no actual size increase here
(we're shrinking NodeFlags by 1 byte, and adding 1 byte of EventState). Moving the NodeFlags to the end
of Node and the EventState bits to the beginning of Element doesn't seem to helper either.
This is probably a padding issue that's worth investigating at some point, but given the level
of churn in this code it doesn't seem worth it to fuss to much over this right now.
