Clean up the interface between `Token` in `models/tokens` and `Token` in `core/response`.

I think it would make the code easier to understand and maintain if these two types had slightly different names and there would be some kind of `from_token` and `to_token` methods that convert them into one another.

Furthermore, use `Self` instead of `Token` in `core/response` where applicable, to further differentiate between the two.