Generational Arena DOM

This is an implementation of the DOM used in html5ever using generational_indextree, an implementation of indextree using generational arenas.

Using an indextree for the DOM makes mutation much simpler, as the docs for indextree state:

This arena tree structure is using just a single Vec and numerical identifiers (indices in the vector) instead of reference counted pointers like. This means there is no RefCell and mutability is handled in a way much more idiomatic to Rust through unique (&mut) access to the arena.

However, indextree suffers from the ABA problem, which we can solve via using generational-arenas instead of Vec based arenas.