refactor: have Index::next_key avoid cloning Arc

nomt/src/beatree/index.rs

@@ -23,12 +23,12 @@ impl Index {
         self.first_key_map.get_prev(&key).map(|(sep, b)| (sep.clone(), b.clone()))
     }
 
-    /// Get the first branch with separator greater than the given key.
-    pub fn next_after(&self, key: Key) -> Option<(Key, Arc<BranchNode>)> {
+    /// Get the first separator greater than the given key.
+    pub fn next_key(&self, key: Key) -> Option<Key> {
         self.first_key_map
             .range(RangeFromExclusive { start: key })
             .next()
-            .map(|(k, b)| (*k, b.clone()))
+            .map(|(k, _)| *k)
     }
 
     /// Remove the branch with the given separator key.

nomt/src/beatree/ops/update/branch_stage.rs

@@ -262,7 +262,7 @@ fn reset_branch_base_fresh(
         return;
     };
 
-    let cutoff = bbn_index.next_after(key).map(|(k, _)| k);
+    let cutoff = bbn_index.next_key(key);
 
     branches_tracker.delete(separator, branch.bbn_pn().into(), cutoff);
 

nomt/src/beatree/ops/update/leaf_stage.rs

@@ -43,7 +43,7 @@ fn indexed_leaf(bbn_index: &Index, key: Key) -> Option<(Key, Option<Key>, PageNu
     let cutoff = if i + 1 < branch.n() as usize {
         Some(get_key(&branch, i + 1))
     } else {
-        bbn_index.next_after(key).map(|(cutoff, _)| cutoff)
+        bbn_index.next_key(key)
     };
 
     Some((separator, cutoff, leaf_pn))