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Add ability to transition type.

One of the key features about using a Union type to represent a state
machine is that you must have a convenient way to transition from one
type to the next.

Previously, we were doing it by adding by returning a freshly minted
state:

```javascript

change() {
  return Color.Red.create();
}
```

This works, but is only quasi-microstate-y. Really, the microstate way
would be to provide a transition method.

This commit does just that. Instead of returning a new instance of
microstate, they `type` parameter is now smart, and hase a transition
method to every other possible member (or State) of the union type. So
the above code can be re-written:

```javascript
change() {
  return this.type.toRed();
}
```

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