Function lehmer_gcd 

fn lehmer_gcd(
    a_in: &BigInt,
    b_in: &BigInt,
    extended: bool,
) -> (BigInt, Option<BigInt>, Option<BigInt>)

lehmerGCD sets z to the greatest common divisor of a and b, which both must be != 0, and returns z. If x or y are not nil, their values are set such that z = ax + by. See Knuth, The Art of Computer Programming, Vol. 2, Section 4.5.2, Algorithm L. This implementation uses the improved condition by Collins requiring only one quotient and avoiding the possibility of single Word overflow. See Jebelean, “Improving the multiprecision Euclidean algorithm”, Design and Implementation of Symbolic Computation Systems, pp 45-58. The cosequences are updated according to Algorithm 10.45 from Cohen et al. “Handbook of Elliptic and Hyperelliptic Curve Cryptography” pp 192.