Macros to build async version of an API

Example

use build_async::{_async, _await};

#[_async] fn foo() {
    _await!(boo());
    _await!(x.zoo());
}

Will produce the following code:

fn foo() {
    boo();
    x.zoo();
}
async fn foo_async() {
    boo_async().await;
    x.zoo_async().await;
}

It works similarly with traits.