feat: generic identifier

[EdJoPaTo]

closes #24

[EdJoPaTo]

This approach where the identifier needs to be unique among its item siblings basically screams for a Map<Identifier, TreeItem>.

Both HashMap and BTreeMap are not ordered (see https://stackoverflow.com/questions/30243100/how-do-i-sort-a-map-by-order-of-insertion). To keep some sort of order with the items this would require either PartialOrd on Identifier or some thing like this: items: HashMap<Identifier, TreeItem>, order: Vec<Identifier>.

Both approaches don't seem easy to use to me.

[EdJoPaTo]

I think going this route for now is the best approach. There might be better options which other people can suggest after playing around with this new release. (Feel free to open a new issue discussing your idea!)