PHP 8.5: Fix "float-string is not representable as an int" warning in BigInteger::toInt()

Since PHP 8.5:

```php
$bi = BigInteger::of('1405006117752879898543142606244511569936384000000000');

$bi->toInt();
```

Result in:

``` shell
Warning: The float-string "1405006117752879898543142606244511569936384000000000" is not representable as an int, cast occurred
```