Add fiber compatibility mode for PHP < 8.1

[clue]

This changeset adds a limited fiber compatibility mode for PHP < 8.1. This means you can now simply use await() in your controller code on any supported PHP version without having to wrap anything in any explicit async calls anymore. 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
    );
});

Builds on top of #116, #117, and #118