Implement isPrime function

[code28]

I implemented isPrime and isProbablyPrime.

/// Return whether the receiver is definitely prime
/// About `reps` see `isProbablyPrime`
///
/// - Parameter `reps`: number of probabilistic prime tests
/// - Parameter `secure`: whether answer is definite (`true`) or probabilistic (`false`)
/// - Returns: `true` iff receiver is (probably or definitely) prime, `false` otherwise
public func isPrime(rounds: Int = 15, secure: Bool = false) -> Bool {
    let isProbPrime = isProbablyPrime(rounds: rounds)
    return secure ? isProbPrime == 2 : isProbPrime > 0
}

/// Return if the receiver is probably prime
/// This function performs some trial divisions,
/// then `reps` Miller-Rabin probabilistic primality tests.
/// A higher `reps` value will reduce the chances of a non-prime
/// being identified as “probably prime”.
/// Reasonable values of reps are between 15 and 50.
///
/// - Returns: `2` (definitely prime), `1` (probably prime), `0` (definitely non-prime)
public func isProbablyPrime(rounds: Int = 15) -> Int {
    return Int(__gmpz_probab_prime_p(&mpz, Int32(rounds)))
}

This PR depends on #1, therefore those commits are included.