Generic function support

Consider the code below

```ts
const echo = quansync({
  sync: (v: any) => v,
  async: async (v: any) => v,
}) as {
  <T>(val: T): QuansyncGenerator<T> & Promise<T>
  sync: <T>(val: T) => T
  async: <T>(val: T) => Promise<T>
}
```