Add concurrent() function

[rela589n]

This PR adds a concurrent() function for creating a fixed set of concurrent operations:

function concurrent(array $closures): array
{
    return \array_map(async(...), $closures);
}

When combined with other functions through pipe operator, it produces easy-to-comprehend concurrent code:

$results = [
    fn () => $httpClient->request(new Request('https://google.com', 'HEAD')),
    fn () => $httpClient->request(new Request('https://bing.com', 'HEAD')),
] |> concurrent(...) |> await(...);

$firstReachableApi = [
    fn () => $httpClient->request(new Request('https://a.api.com', 'HEAD')),
    fn () => $httpClient->request(new Request('https://b.api.com', 'HEAD')),
] |> concurrent(...) |> awaitAny(...);

[xpader]

No meaning fot this..

[rela589n]

@trowski , @kelunik , once this is merged, I think I'll open a dedicated PR for settle() so that #467 can be closed. concurrent() is a good thing for settle() README examples, so I'll linger opening a new PR till this is merged.

[kelunik]

Do we want to support iterable here?

[rela589n]

It's not a problem with me to make it iterable / array,
yet the decision is yours, guys.