Previously `afterRender` and `afterNextRender` allowed the user to pass
a phase to run the callback in as part of the `AfterRenderOptions`. This
worked, but made it cumbersome to coordinate work between phases.

```ts
let size: DOMRect|null = null;

afterRender(() => {
  size = nativeEl.getBoundingClientRect();
}, {phase: AfterRenderPhase.EarlyRead});

afterRender(() => {
  otherNativeEl.style.width = size!.width + 'px';
}, {phase: AfterRenderPhase.Write});
```

This PR replaces the old phases API with a new one that allows passing a
callback per phase in a single `afterRender` / `afterNextRender` call.
The return value of each phase's callback is passed to the subsequent
callbacks that were part of that `afterRender` call.

```ts
afterRender({
  earlyRead: () => nativeEl.getBoundingClientRect(),
  write: (rect) => {
    otherNativeEl.style.width = rect.width + 'px';
  }
});
```

This API also retains the ability to pass a single callback, which will
be run in the `mixedReadWrite` phase.

```ts
afterRender(() => {
  // read some stuff ...
  // write some stuff ...
});
```