Add support for async / await #84

In modern javascript, there is async and await new keywords that allows to write asynchronous code that looks like synchronous code. For instance,

async function fetchPosts() {
    let response = await fetch('/api/posts');
    if (response.status === 200) {
         let posts = await response.json();
         return posts;
    }
}

Is it possible to have support for that? I don't how to port my framework to racketscript without this (see. #61).

The above should be possible to be written in racketscript using a syntax that looks like the following:

(define-async (fetch-posts
 (let ((response (await (fetch "/api/posts"))))
       (if (= (response-status response) 200)
           (await (response-json response)))))

It should also be possible to define async lambda.