Followers are a tool for bringing Functional Reactive Programming ideas to a ClojureScript stack, e.g. for use with
React. A follower is a data transformation function raised to operate over state, where state can be encapsulated by
Atoms, or other followers. When state changes, the follower will recompute its internal value and notify listeners
so they can respond to the change.

When a state update occurs, the API performs a topological sort then updates followers in order to ensure that each
follower will only recompute once per state change (each listener will only be called once).

Followers are stateful objects with references to their state sources, so they need to be cleaned up manually when
they are no longer required (JavaScript Weak References are not widely available).