Flesh out directions renaming

- use more readable `.endChild` instead of `.ch` (and for consistency, `.end` instead of `.ends`)
- similarly, use more typeable `.insDirOf` instead of `.insertAdjacent`, `.insAtDirEnd` instead of `.appendDir`
- replace all variable names and comments using the outdated "prev / next / first / last / `firstChild` / `lastChild` / `insertBefore` / `insertAfter` / `prependTo` / `appendTo`" nomenclature with left/right-derived directional terminology

Not quite ready for merging, I still want to use a Pjs class instead of the `jQ{ins{DirOf,AtDirEnd},getExtreme}` method-wannabes.