a way to subscribe to external stores

[tombl]

something like

render() {
	const value = use(this, store, {
		snapshot(store) {},
		subscribe(store, cb) { return () => { /* unsub */ } }
	})
	return html`...`
}

relies on object identity of the passed store in order to detach, so it must be memozied. also probably should be keyed on the subscriber too.

keeps a list of every store used in this render, and compares it to the list from last time. any stores no longer used are automatically unsubscribed.