<issue_start><issue_comment>Title: Document performance characteristics
username_0: It'd be great to know how `TreeId`s and `TreeMeta`s should be implemented to get the best performance out of the `Tree`.

One thing that came to mind was how to handle data with internals ids - should I use the data as the `TreeId` and leave the metadata as `()`? Or is it better to duplicate the id and create a `Tree<Id, DataWithCopyOfId>`? It'll depend on how often the two types are cloned, and in which situations.