Extend the node accessor to create composite nodes

[shiftyp]

The idea here would be to not limit nodes to a particular tree shape, and allow you to create new nodes that are composites of other nodes in the tree. The api might look something like this:

const state = createState({
  foo: {
    bar: 1,
    baz: 2
  },
  quz: 3
});

const composite = state.compose({
  baz: 'foo.baz',
  qux: 'qux'
});

Then you could reduce this node using the defined keys, and the state of both the composite node, the original nodes, and their original parent nodes would be updated.

const action = createAction();

composite.reduce(action, (compositeState, val) => {
  return {
    baz: compositeState.baz + val,
    qux: compositeState.qux + val  
  };
});

state('foo.baz').asObservable()
  .subscribe(val => console.log(val));

state.connect();
// 2
action(3);
// 6

The main use for this feature wouldn't be reducing the composite node, but rather subscribing to the composite node. So if you have a component in your view system that requires widespread elements of state you could create a composite observable to observe when those properties change.