Start on maintaining an insertions tree

I'm correctly assigning fragment ids to all fragments in the fragments tree, but I have a randomized test failure when making sure that the insertions tree matches the state of the fragments tree.

crates/text/src/tests.rs

@@ -51,7 +51,7 @@ fn test_random_edits(mut rng: StdRng) {
     );
 
     for _i in 0..operations {
-        let (old_ranges, new_text, _) = buffer.randomly_edit(&mut rng, 5);
+        let (old_ranges, new_text, _) = buffer.randomly_edit(&mut rng, 1);
         for old_range in old_ranges.iter().rev() {
             reference_string.replace_range(old_range.clone(), &new_text);
         }
@@ -78,6 +78,27 @@ fn test_random_edits(mut rng: StdRng) {
             TextSummary::from(&reference_string[range])
         );
 
+        // Ensure every fragment is ordered by locator in the fragment tree and corresponds
+        // to an insertion fragment in the insertions tree.
+        let mut prev_fragment_id = Locator::min();
+        for fragment in buffer.snapshot.fragments.items(&None) {
+            assert!(fragment.id > prev_fragment_id);
+            prev_fragment_id = fragment.id.clone();
+
+            let insertion_fragment = buffer
+                .snapshot
+                .insertions
+                .get(
+                    &InsertionFragmentKey {
+                        timestamp: fragment.insertion_timestamp,
+                        split_offset: fragment.insertion_offset,
+                    },
+                    &(),
+                )
+                .unwrap();
+            assert_eq!(insertion_fragment.fragment_id, fragment.id);
+        }
+
         if rng.gen_bool(0.3) {
             buffer_versions.push((buffer.clone(), buffer.subscribe()));
         }