// 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 = a*x + b*y.
// 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.