sum_tree: Implement recursive Sumtree::find, use it over Cursor::seek if possible (#40700)

Reduces peak stack usage in these functions and should generally be a
bit performant.

Display map benchmark results
```
To tab point/to_tab_point/1024
                        time:   [531.40 ns 532.10 ns 532.97 ns]
                        change: [-2.1824% -2.0054% -1.8125%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high severe

To fold point/to_fold_point/1024
                        time:   [530.81 ns 531.30 ns 531.80 ns]
                        change: [-2.0295% -1.9054% -1.7716%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 3 outliers among 100 measurements (3.00%)
  2 (2.00%) high mild
  1 (1.00%) high severe
```

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/multi_buffer/src/multi_buffer.rs

@@ -6140,9 +6140,8 @@ impl MultiBufferSnapshot {
         } else if id == ExcerptId::max() {
             Locator::max_ref()
         } else {
-            let mut cursor = self.excerpt_ids.cursor::<ExcerptId>(());
-            cursor.seek(&id, Bias::Left);
-            if let Some(entry) = cursor.item()
+            let (_, _, item) = self.excerpt_ids.find::<ExcerptId, _>((), &id, Bias::Left);
+            if let Some(entry) = item
                 && entry.id == id
             {
                 return &entry.locator;