Refactor zipper

[NickNeck]

Hi, I have made some refactoring to the zipper.

It could be too much for a PR, and no changes are necessary. However, it shortens the code and makes it a bit more readable—not that it is not readable in the current version.

Changes:

• the function subtree was simplified and inlined to use it in the traverse* functions.
• a private function into was created to use it in the traverse functions.
• the private function prev_after_remove was removed because it was a duplicate of do_prev.
• added some function headers to get the right argument names in the generated docs.
• added some highlighting in the docs.

[doorgan]

This is really nice, thanks for all these improvements! 💜