add simplex benchmark to perf2

This should help prevent regressions like #3142. The code is adapted from my **[SimplexBenchmarks]** project.

[SimplexBenchmarks]: https://github.com/mlubin/SimplexBenchmarks

Before the fix to #3142 (f2f6351)
```
simplex		124.45497512817383
```

After the fix (5fb39b86f8):
```
simplex		36.93795204162598
```