async / await support - the function based implementation

[engelant]

I just think the support of async/await can be divided into two steps:

1. First implement async/await with library functions; 2. Implement related syntactic sugar.

We should open a new issue.

Originally posted by @Skiars in https://github.com/Skiars/berry/issues/65#issuecomment-669629602

[skiars]

Good. Recently I am trying js asynchronous operation, I think this will give some inspiration.