Extend random_prime() to be able to generate primes of any bit size.
Extend random_prime() to be able to generate primes of any bit size.

bits <= 1 -> error
bits == 2 -> choose 2 or 3 at random
bits == 3 -> choose 5 or 7 at random
bits == 4 -> choose 11 or 13 at random
bits >= 5 -> procedure used previously.

Tested by running random_prime() with random bit sizes <= 16 until it had
generated all <= 16 bit primes.

Rohde-Schwarz-Cybersecurity/botan,randombit/botan,Rohde-Schwarz-Cybersecurity/botan,webmaster128/botan,randombit/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,randombit/botan,randombit/botan,Rohde-Schwarz-Cybersecurity/botan,webmaster128/botan,randombit/botan,webmaster128/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan