Allow converting async block to Traits

[pookjw]

This PR allows converting async block to RxSwift Traits - Infailable, Single, Maybe, Completable, Signal, Driver.

Single.from {
    await UIImage(named: "background")?.byPreparingThumbnail(ofSize: CGSize(width: 10, height: 10))
}
.compactMap { $0 }
.asObservable()
.bind(to: imageView.rx.image)
.disposed(by: disposeBag)