Add necessary methods to support finding nodes/versions resulting from a split

Can be used in a transactional system to update the absent set after a write-my-reads insert that results in a split.

I can squash the commits to make the git log nicer?
Hi hi hi!

At this point, n_ is being split "vertically" into a new trie layer. The actual new value will be stored in a lower-level layer; a couple lines below we assign n_ again. I don't think you want to set newv_; you want to append n_ and n_->full_unlocked_versioned_value() to newnodes_.