README's `generatePrime(_ width: Int)` performances

[nathanfallet]

The README's generatePrime(_ width: Int) function is very slow for big numbers (can take up to 10s to generate two primes for a RSA key)

func generatePrime(_ width: Int) -> BigUInt {
    while true {
        var random = BigUInt.randomInteger(withExactWidth: width)
        random |= BigUInt(1)
        if random.isPrime() {
            return random
        }
    }
}

Any way to generate primes quicker?

[kecson]

The generatePrime(2048) method also take up to a few minutes to generate prime numbers for Paillier Homomorphic Encryption PrivateKey