Maintain cursor stack's position correctly when ascending the tree

This fixes a bug that could cause the cursor to incorrectly report its
start when using `slice` or `seek_forward`, and then calling `prev`. We
didn't notice this because we were not testing those three methods
together.

I suppose this could explain some of the panics we've observed because
we do use `slice`/`seek_forward` followed by `prev` calls in production.

crates/sum_tree/src/cursor.rs

@@ -438,6 +438,7 @@ where
                 } => {
                     if ascending {
                         entry.index += 1;
+                        entry.position = self.position.clone();
                     }
 
                     for (child_tree, child_summary) in child_trees[entry.index..]