Support generators

This sort of feature would help us develop features like `Array.prototype.entries`, `Array.prototype.values`, and other features that require generators.

For instance:

```ts
class Array<T> {
  *values(): Iterator<T> {
    var length = this.length_;
    for (let i = 0; i < length; i++) {
      yield this.__unchecked_get(i);
    }
  }
}
```