pub(crate) fn change_type<U: Serialize + DeserializeOwned>(self) -> TemplateStateWithType<U> {
        TemplateStateWithType {
            state: self.state,
            ty: PhantomData,
        }
    }
Transform this into a [`TemplateStateWithType`] with a different type.
Once this is done, `.to_concrete()` can be used to get this type out
of the container.
packages/perseus/src/state/template_state.rs