I've managed to construct a prototype of this "stateless" iterator.

I've only implemented the in-order case and only applied it in the overlays_in function.

The only real trouble I had was with pushing the offset to the children
during the tree navigation in some kind of sensible way.  In the end I
gave up and simply pasted calls to the corresponding function "all over
the place".  It seems to work, at least buffer-tests are passing.

Andreas