Remove component-emitter dependency

[maxkueng]

Removes the component-emitter dependency and replaces it with a built-in list of listeners.

Reasons

• Reduce the build size
• Emitter was used for just a single event type

API changes (breaking)

Previous:

const state = new Bibimbap();
function onCommit(tree, prev, cursor) {
  // stuff
}
state.on('commit', onCommit);
state.off('commit', onCommit);

New:

const state = new Bibimbap();
const unsubscribe = state.subscribe((tree, prev, cursor) => {
  // stuff
});
unsubscribe();