Proposal: `await.all {...}` for parallelism

[manuelbarzi]

>
> Hi Manuel! Would you mind explaining the added value of your proposal? I
> am only seeing it being more verbose, but I've not found any added
> functionality or benefit from it
>

the proposal combines parallel and series async / await and could resolve
complex trees, like the following example:

```
// NOTE async {} = asynchronous scope (it groups parallel awaits => `await||`)
// NOTE p? = function call that returns a promise (? = just and index)

async {
    const x1 = await|| p1()
    const x2 = await p2(x1)
    const x3 = await|| p3()
    const x10 = await p10(x2, x3)

    let x4, x5, x6

    async {
        x4 = await|| p4(x1, x2)
        x5 = await|| p5(x2, x3)
        x6 = await p6(x4, x5, x10)
    }

    let x7, x8, x9

    async {
        x7 = await|| p7(x4, x6)
        x8 = await p8(x6, x7)
        x9 = await|| p9(x5, x6)
    }

    await p11(x8, x9)
}

// it would resolve a tree of parallel and series like following with
traditional promises

Promise.resolve()
    .then(() => Promise.all([p1, p3]))
    .then((x1, x3) =>
        p2(x1)
            .then(x2 =>
                p10(x2, x3)
                     .then(x10 => {
                        let x4, x5, x6

                        return Promise.resolve()
                            .then(() => Promise.all([p4(x1, x2), p5(x2, x3
)]))
                            .then(results => [x4, x5] = results)
                            .then(() => p6(x4, x5, x10))
                            .then(x6 => {
                                let x7, x8, x9

                                return Promise.resolve()
                                    .then(() => Promise.all([p7(x4, x6), p9(
x5, x6)]))
                                    .then(results => [x7, x9] = results)
                                    .then(() => p8(x6, x7))
                                    .then(_x8 => x8 = _x8)
                                    .then(() => p11(x8, x9))
                            })
                     })
            )
    )
```
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