Move `sqrt()` out of `paillier::SecretKey`

[fjarri]

It seems like it should be hosted somewhere at the uint level. It would be nice to move it as far as crypto-bigint, but we are relying on the primes being of specific form (== 3 mod 4), and also we would have to generalize it to an arbitrary number of primes in the RNS instead of the fixed 2, so it may be too much work.