Introducing generators

Export rows one by one on large dataset to avoid memory_limit problem, thanks to generators (using yield and yield from).

function usersGenerator() {
    yield from User::chunk(200, function($users) {
        foreach($users as $user) {
            yield $user;
        }
    });
}

// Export consumes only a few MB, even with 10M+ rows.
(new FastExcel(usersGenerator()))->export('test.xlsx');