shootout-mandelbrot: Either 10-20% or 80-100% improvement.

Not sure if we're accepting SIMD usage in the main tree, so the first commit is just unrolling the loop a bit to calculate two bits at a time, this gives a 2second speedup from 12s => 10s on my i5.

The second commit using SIMD which brings it down to about 6s. It also precomputes initial values (which doesn't help much, but it is noticeable).

If we're allowing SIMD in the benchmarks, I can squash to the 2nd commit, otherwise I can kill the 2nd commit and leave the first.