Use negated filter for reject

Since `reject` is just the inverse of `filter`, there's no need to re-invent the wheel.

Also, this is a micro optimization against checking `$callback instanceof Closure` in every iteration.

---

Another option would be to do a diff:
```php
if ($callback instanceof Closure)
{
    return $this->diff($this->filter($callback));
}
```

which is cleaner, but I'm afraid it might slow down this operation when dealing with bigger collections.