Eliminate O(n) tree-order sorted insert on every add() by simply
appending elements and deferring order resolution to lookup time. Cache
the first-in-tree-order element in get() so repeated getElementById()
calls avoid repeated subtree traversal.
Improves performance on YouTube where add() was hot in profiles while
scrolling through comments.
With this change we maintain a data structure that maps ids to
corresponding elements. This allows us to avoid tree traversal in
getElementById() in all cases except ones when lookup happens for
unconnected elements.
