Refactoring

[lo48576]

• Improve traversing iterators.
  • Iterator internals are hidden from other modules.
  • Lowered nest level of the implementations.
• Add types to represent and manipulate sibling nodes range.
  • SiblingsRange is a range of sibling nodes.
  • DetachedSiblingsRangeis a detached siblings node range.
    • Tirst node of the range does not have a previous sibling, and the last node of the range does not have a next sibling.
• Add helper functions to manipulate nodes relations.
  • connect_neighbors() connects the given nodes.
  • insert_with_neighbors() inserts the given node among the given neighbors.
• Rewrited various insertions, detach, and remove methods of NodeId.
  • They use SiblingsRange internally, and the codes are now very easy to read.
• Added more assetions to check consistency.

[saschagrunert]

Phew, that was a bunch of review. :) Thanks again, generally LGTM 🙏👍