-O2 vs -O3 compiler optimization level

I haven’t measured compile times yet, but it would be surprising if compile-time was slower and it seems like runtime is better. I haven’t measured compile-time yet because these measurements were initially done before the perf branch of cmdstan had compile-time measurements and I haven’t had an idle machine to run benchmarks on since then, but I’ll try to make sure I do this before I start a job and get busy.

Also, are there any other models you’d suggest running before I submit a PR for this? The PR should be trivial, but I want to make sure I’m not missing something obvious from the measurement side.