fix: treepy-leftmost, treepy-rightmost

[jepsoniada]

The Issue

Functions treepy-leftmost and treepy-rightmost doesn't represent left and right hand side like treepy-left or treely-right does.
This affects reconstruction of data whenever it's changed.
Most affected is treepy-rightmost as it reverses whole lhs when going back to root.

Example

(let ((data (treepy-list-zip (list 1 2 (list 3 4)))))
  (treepy-root (treepy-append-child (treepy-rightmost (treepy-down data)) 5)))
;; return (2 1 (3 4 5))
;; expected (1 2 (3 4 5))

What's Changed

Provided patch brings behavior of these function closer to their less specialized counterparts and adds lacking test coverage for them.