Access values of async variables as they become available

[crontab (Hovik Melikyan)]

@cukr

without introducing anything new:

async let u = {
    let x = await f()
    // use x
    return x
}()

Yes and as a side note, make it look cooler with this idea from Arbitrary Block Evaluation - #15 by crontab

async let u = get {
    let x = await f()
    // use x
    return x
}