This is a thing I've been meaning to set up for a while: it's a
pull-based search system (that is, the caller takes each step of the
search manually, rather than providing a callback), which lets the
caller inspect every step of the search, including the index of each
candidate element in the tree. This allows flexible kinds of query
that play the element and its index off against each other.
I've also rewritten the existing findrelpos234() search function using
the new one as a primitive, because that simplifies it a lot!