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

...and $min->randomPrime($max) as well
This has been updated. That said because I do do `$this->assertFalse($rand1->equals($rand2));` I have to do generate two random numbers anyway. As such I think it makes sense to test two methods in one function. eg. for $rand one do `$min->random($max)` and for the other do `$max->random($min)`.