Module reactivity

Simple, explicit, push-based reactivity system.

Principal functionality:

• Reactive.of(...): make a value reactive.
• Reactive.nest(...): make an object or array and their direct properties/elements reactive.
• Reactive.derive(tracker => reactive1.get(tracker) + reactive2.get(tracker)): Derive a reactive value from other reactive values.
• reactive.get(tracker): Get the underlying value and track this dependency (important with derivatives ⤴️).
• reactive.unwrap(): Get the underlying value without dependency tracking.
• reactive.consume(value => ...): register a consumer function that gets called with the underlying value immediately and after every update.
• reactive.set(...): Update the underlying value.
• reactive.patch(...): Where possible, only update the differences with the given reference value.

Reactive is the base abstraction of a reactive entity. Its subclass ReactiveValue wraps an actual value, while ReactiveDerivative represents a derived value. ReactiveArray is a specialization offering convenience methods to work with arrays and ReactiveObject is the same for objects.

The tests contain more usage examples.