This PR shares the driver code from the Sym-based mvcgen benchmarks. It
also moves the `simp only [loop, step]` call out of the measured
section, so that we measure purely the overhead of VC generation.
The new benchmark results are as follows. All measurements for n=1000:
```
baseline_add_sub_cancel: 719.318425 ms, kernel: 382.708178 ms
vcgen_add_sub_cancel: 306.883079 ms, kernel: 455.050825 ms
vcgen_deep_add_sub_cancel: 543.350543 ms, kernel: 896.926298 ms
vcgen_get_throw_set: 669.566541 ms, kernel: 60754.202714 ms
```
Note that `vcgen_add_sub_cancel` sped up by 100% because we no longer
measure unfolding `loop` and `step`. The baseline didn't speed up as
much because it unfolded in the same `Sym.simp` call that also does
other rewrites, so there was no `simp` pass that could be eliminated.
f084ce1497