Built-in support for fibers on PHP 8.1+ with stable reactphp/async

[clue]

This changeset adds built-in support for fibers on PHP 8.1+ with stable reactphp/async v4. The fiber compatibility mode for PHP < 8.1 now also uses stable reactphp/async v3. This makes building and consuming async APIs easier than ever before.

$app->get('/book/{isbn}', function (Psr\Http\Message\ServerRequestInterface $request) use ($db) {
    $isbn = $request->getAttribute('isbn');
    $result = await($db->query(
        'SELECT title FROM book WHERE isbn = ?',
        [$isbn]
    ));

    assert($result instanceof React\MySQL\QueryResult);
    $data = $result->resultRows[0]['title'];

    return React\Http\Message\Response::plaintext(
        $data
    );
});

Most notably, this changeset adds required dependencies to make sure the fiber API works across all installations. It does not otherwise affect existing installations. All changes have been performed upstream in the reactphp/async component, so we can take advantage of this finally now that it is available as a stable release via https://github.com/reactphp/async/releases/tag/v4.0.0.

Builds on top of #117 and #128