Add utility to allow type-safe `yield`s

Probably something like this:
```ts
function* waitFor<T>(yieldExpr: () => Promise<T>): Generator<Promise<T>, T, T> {
    const unwrapped = yield yieldExpr;
    return unwrapped;
}
function* task() {
    const x = yield* waitFor(() => Promise.resolve(5));
    const y = yield* waitFor(() => Promise.resolve('hello'));
}
```