[RFC]Support async generator as Effect

[Brooooooklyn]

RxJS is great as process management, but sometimes our Effect dosen't need too much async management ability. So I really want a simpler way to provide async management ability.

@GeneratorEffect()
async fetchUserInfo(payload: T) {
 const response = await client.query({ ... })
 try {
 yield this.getActions().setUserInfo(assertSuccess(response))
 } catch (e) {
 yield this.getActions().getUserInfoFailed(e)
 }
}