R2.2.0

Release R2.2.0

Summary of changes:

• Prevent cycles by rejecting attempts to add an ancestor as a child.
• Ensure remove_all! detaches children by clearing parent links.
• Guard rename_child against sibling name collisions.
• Harden binary child assignment (set_child_at) with proper index errors and cleanup.
• Make traversals resilient to missing children by skipping nils in postorder/breadth.
• Fix each_level to return a level-wise enumerator when no block is given.
• Improve to_s to show <Empty> for nil content.
• Bump version to 2.2.0 and update History/API-CHANGES.