message: Use Iterator::all() in place of Iterator::fold()
diff: @@ -145,7 +145,7 @@ impl CommitmentTree {
  self.left.is_some()
  && self.right.is_some()
  && self.parents.len() == depth - 1
  - && self.parents.iter().fold(true, |acc, p| acc && p.is_some())
  + && self.parents.iter().all(|p| p.is_some())
  }
  /// Adds a note to the tree. Returns an error if the tree is full.
  