Ensure constant bit size of Paillier modulus

[fjarri]

Since the current crypto-primes behavior is to only set the MSB, the bit size of the product of two primes of bit size n can have the size of 2n or 2n-1 bits. With entropyxyz/crypto-primes#69 merged we can now set the second MSB as well, making the product bit size constant. This should be done in rsa.rs, and the modulus size conditions made more strict (look for MODULUS_BITS - 2 lines).