added Parakeet benchmarks to arc_distance and pairwise, updated cold/warm timings

Hey Olivier, 

Sorry if this is too much stuff in one pull request, I can split things up if you'd like. 

There are a few changes here:
- parallel Parakeet implementation of arc distance 
- parallel Parakeet implementation of pairwise distances (surprisingly slow!)
- run_benchmarks.py: keep the median warm timings instead of the min
- run_benchmarks.py: only disables cold/warm timings for pure Python functions