Mini Redux store

Implement createStore(reducer, initialState), a tiny version of a Redux store. A reducer is a function (state, action) => newState that computes the next state from the current one and an action object.

The store has three methods:

• getState() returns the current state.
• dispatch(action) sets the state to reducer(currentState, action). Then it calls every listener with no arguments and returns action.
• subscribe(listener) registers a listener and returns an unsubscribe function.

Rules for listeners:

• They run after the state has been updated, in the order they subscribed.
• Subscribing the same function twice registers it twice. Each unsubscribe removes only its own registration, and calling it again does nothing.
• If a listener subscribes or unsubscribes during a dispatch, the change applies from the next dispatch. The current dispatch calls exactly the listeners that were registered when it started.