Integrate with criterion?

It seems as though this both would complement and be improved by integration into criterion:
- instruction counts as regression would be validated by criterion timing
- a timing regression combined with no increase in instr count would be a really useful thing to know (e.g. it might point to a space leak)
- standalone timing tests like the one in the example in the README here are fraught due to compiler optimizations; criterion is meant to solve this

What are your thoughts? I suppose it would need to be conditional on linux OS and be optional due to the permissions issues you mention