Creating venvs takes very long on Windows

[gvanrossum]

On Windows, creating the venvs takes a long time (20-30 minutes). And when re-running, it takes a little less long but still very long (5-10 minutes?).

I wonder if we couldn't share the venv at least for those benchmarks that don't install third party packages (other than pyperformance and its dependencies -- packaging, pyperf, pyparsing, toml).

Most benchmarks don't depend on anything else, so this would be a huge speedup for starting the benchmarks.

[ericsnowcurrently]

A small portion of this is fixed with #151. I should have a PR for the rest up tomorrow.

[ericsnowcurrently]

@gvanrossum, sorry for the delay. I've pushed up some changes that should help speed things up. If that isn't enough, I have a few other ideas we can try out.

[gvanrossum]

Good enough now.