Update async API to fix state closure bug - 0.4.0

[msmith-techempower]

Addresses the issue where two hook actions are invoked simultaneously and having state closed for both, thus the non-first reducer to be fired with stale state (overriding the n-1th state mutations).

As a side-effect, it means we cannot directly support async HookActions that mutate state. Instead, we now support async by enforcing that the promise will resolve with a reducer callback function. In this way, async actions can be guaranteed that the state passed to their reducer callback will not be stale.

Example:

const contract = {
  async foo() {
    const foo = "foo";
    await new Promise(resolve => setTimeout(() => resolve(), 3000));

    return state => ({
      ...state,
      foo
    });
  }
};

Invoking actions.foo() will return immediately and not mutate the current state. Internally, the promise will block for 3 seconds, then return a state reducer mutates the current state to have a property foo with value "foo".

The last change in this PR is that this.state is no longer available (it was closed anyway) and all HookActions must use the passed state variable (always the last arg).