[std] Promise._map: run async functions sequentially and store results

[wmerfalen]

• Promise._sleep(n)
• Proimse._map(arr, fn)
• Proimse._parallel(n, arr, fn)

Run fn on each element of arr, one at a time, and give back a sequentially ordered list of results.

// do something like `throw new Error('break');` to stop
Promise._map = async function (arr, fn) {
    let results = [];
    await arr.reduce(async function (promise, el, i) {
        await promise;
        let result = await fn(el, i, arr);
        results.push(result);
    }, Promise.resolve());
    return results;
};