Faster Euclidean algorithm

[mratsim]

This implementation I made in 
<https://github.com/SciNim/impulse/blob/49b813232507470a047727712acda105b84c7815/impulse/fft/factorization.nim#L23-L39>
 is faster on my machine
    
    
    func gcd_impulse(u, v: int): int =
      # From 
https://github.com/SciNim/impulse/blob/49b813232507470a047727712acda105b84c7815/impulse/fft/factorization.nim#L23-L39
      if u == 0: return v
      if v == 0: return u
      let shift = countTrailingZeroBits(u or v)
      var u = u shr u.countTrailingZeroBits()
      var v = v
      while true:
        v = v shr v.countTrailingZeroBits()
        if u > v:
          swap(u, v)
        v = v - u
        if v == 0:
          return u shl shift
    
    
    Run

gist: <https://gist.github.com/mratsim/afb82da37287fb79dcd6825acec23f65>
    
    
    gcd in stdlib: 982 micro second
    gcdLAR:        1195 micro second
    gcdLAR2:       1648 micro second
    gcdLAR3:       1479 micro second
    gcdLAR4:       871 micro second
    gcdSub:        871 micro second
    gcdSub2:       1081 micro second
    gcd_impulse:       857 micro second
    
    
    Run