BigInteger: make it so you can do $min->random($max)

...and $min->randomPrime($max) as well
This is actually interesting. Because that won't work due to `$compare = $max->compare($min);`. Found via https://scrutinizer-ci.com/g/phpseclib/phpseclib/inspections/8c1fbe77-91e4-4019-89a8-492de39f3003/issues/files/phpseclib/Math/BigInteger.php?status=new&orderField=path&order=asc