neon: Investigate possible bug in `im` crate

with the following IM patch layer bench normally completed:

diff --git a/src/nodes/btree.rs b/src/nodes/btree.rs
index 84f63fa..2e6452e 100644
--- a/src/nodes/btree.rs
+++ b/src/nodes/btree.rs
@@ -235,6 +235,24 @@ impl<A: BTreeValue> Node<A> {
         }
     }
 
+    fn child<'a, BK>(&'a self, key: &BK, index: usize) -> Option<&A>
+    where
+        BK: Ord + ?Sized,
+        A::Key: Borrow<BK>,
+    {
+        match self.children[index] {
+            None if index == 0 => None,
+            None => self.keys.get(index - 1).map(|_| &self.keys[index - 1]),
+            Some(ref node) => node.lookup_prev(key).or_else(|| {
+                if index == 0 {
+                    None
+                } else {
+                    self.child(key, index - 1)
+                }
+            }),
+        }
+    }
+
     pub(crate) fn lookup_prev<'a, BK>(&'a self, key: &BK) -> Option<&A>
     where
         BK: Ord + ?Sized,
@@ -245,11 +263,7 @@ impl<A: BTreeValue> Node<A> {
         }
         match A::search_key(&self.keys, key) {
             Ok(index) => Some(&self.keys[index]),
-            Err(index) => match self.children[index] {
-                None if index == 0 => None,
-                None => self.keys.get(index - 1).map(|_| &self.keys[index - 1]),
-                Some(ref node) => node.lookup_prev(key),
-            },
+            Err(index) => self.child(key, index),
         }
     }

Performance with patched IM and get_prev is the following:

captest_uniform_queries time:   [3.4951 ms 3.5300 ms 3.5703 ms]
captest_rel_dir_query   time:   [188.40 ns 190.01 ns 191.80 ns]
Finished layer map init in 764.03677ms
real_map/uniform_queries
                        time:   [3.7333 ms 3.8060 ms 3.8843 ms]
real_map/get_difficulty_map
                        time:   [19.668 ms 20.244 ms 20.842 ms]
Finished layer map init in 625.197474ms
sequential/uniform_queries
                        time:   [10.334 µs 10.756 µs 11.213 µs]

Seems to be not “times” faster I have replaced next_back() only in one place - in query(&self, key: i128). It seems to me that you wrote that get_prev() is 70% faster. On which workload?