go big int faster increment

bigIntInc.go

// bigIntInc speeds up z.Add(z, one) operations when z > 0
func bigIntInc(z *big.Int) {
	zBits := z.Bits()
	// big.NewInt( 9).Sign() == 1
	// For speed and simplicity this only tries to increment the least significant digits. 
	// Using a base 10 example: 1000+1 only affects the least significant digit.
	// Only when the least significant digit is 9, 1009+1, then more significant digits are affected.
	if z.Sign() > 0 && zBits[0] < math.MaxUint {
		// overflow does not happen due to zBits[0] < math.MaxUint check
		// uint(zBits[0])++ is not allowed by Go
		zBits[0] = big.Word(uint(zBits[0]) + 1)
	} else {
		z.Add(z, one)
	}
}