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

...and $min->randomPrime($max) as well
Yes, $min and $max need to be BigInteger objects. That's true for pretty much every other BigInteger method as well and I'm not doing any sort of `is_object($arg) && strtolower(get_class($arg)) == 'math_biginteger'` check for any of them. And it hasn't really been a huge problem either.

And given that it hasn't been a huge problem I am hesitant to add type checking. That's only gonna slow things down when you're looping a million times to find a prime number (for example). And if it was a huge problem the slowdown incurred by checking the type might be justified but like I said, outside of this issue, no one's mentioned it before. It just doesn't seem like it's that big an issue.