"Fatal error: Cannot redeclare function" when using polyfill and extension together (2.0)

```
Fatal error: Cannot redeclare function Ds\seq() in .../vendor/php-ds/php-ds/src/functions.php on line 7
```

Function declarations should be wrapped with `if (!function_exists(...))`:

```php
if (!function_exists('Ds\\seq')) {
    function seq(iterable $values = []): Seq
    {
        return new Seq($values);
    }
}
```