useObserver should trigger a rerender only if the observed value changed

[guyca]

Background

We should probably add another means to subscribe to Observables. Perhaps add a new subscribeUnique() method and use it from the useObserver hook.

API

useObserver<T>(observable: Observable, options?: {
  comparator: (a: T, b: T) => boolean
});

The default comparator will only perform a shallow comparison and users will be able to provider their own implementations.