BLD: Add 2to3 caching, move the build cache logic into use_build_cache.py

I tested this script all the way back to 0.7.0. The build cache code was a lot of noise
to shove into setup.py, I'm glad to pull it back out.

a "goto_ver.sh" script is now really easy to do, and very useful for benchmarking
and bisecting regressions.