message: Use the `hash_pair()` utility fn in `hash_merkle_tree()`
diff: @@ -163,15 +163,7 @@ impl Digest {
  let leaf_count_bytes = leaves.len().to_le_bytes();
  let raw_root = leaves
  - .tree_fold1(|mut hash_x, hash_y| {
  - let mut hasher = VarBlake2b::new(Digest::LENGTH).unwrap();
  - hasher.update(&hash_x);
  - hasher.update(&hash_y);
  - hasher.finalize_variable(|slice| {
  - hash_x.0.copy_from_slice(slice);
  - });
  - hash_x
  - })
  + .tree_fold1(Digest::hash_pair)
  .unwrap_or(Digest::SENTINEL_MERKLE_TREE);
  Digest::hash_pair(leaf_count_bytes, raw_root)
  