Collapse `prev_sybling` and `last_child` into `prev_cyclic_sybling`

[matklad]

See "Data structures for the document object model" http://www.aosabook.org/en/posa/parsing-xml-at-the-speed-of-light.html section for a neat trick to save one pointer per node while maintaining O(1) operations.

[saschagrunert]

Cool, thank you for the input. I will take this into consideration.

[lo48576]

As I wrote in #38 (comment), I don't like this trick, because it makes things complex for both internal implementation and library users.

To reduce memory consumption, I like another method to make node ID a type parameter, as string-interner crate does (by Symbol trait).
This won't affect core logic, and users can choose balance between memory consumption and number of nodes.