Modify pyperf to display CPU stats and geomean to one decimal place.

[markshannon]

Modern CPUs have a lot of internal counters for things like L1 cache misses, pipeline stalls, etc.
Those counters would be valuable when comparing minor improvements that don't show much speedup.
perf can capture those stats, so we should also capture and display them.

It would also be useful to have geomean to one decimal place. There is quite a difference in significance between a 0.6% speedup and a 1.4% speedup.

[gvanrossum]

Maybe the next thing to do here is to file an issue with https://github.com/psf/pyperf/issues ?