Discriminant calculation using fma

[c42f]

I was reading an interesting article about posits (thanks @simonbyrne!) and came across Kahan's algorithm for calculating discriminant which avoids catastrophic cancellation and looks cheap-ish on hardware with FMA. At some stage we should probably see whether it's a reasonable performance tradeoff for 2x2 _det and related computations:

// computes: ad-bc within +/- 3/2 ulp of exact
double discriminant(double a, double b, double c, double d)
{
  double w = b*c;
  double e = fma(-b,c,w);
  double f = fma(a,d,-w);
  return f+e;
}

Further analysis: http://www.ams.org/journals/mcom/2013-82-284/S0025-5718-2013-02679-8/home.html