Run some specs as benchmarks

We'll need to keep track of performance, and know what to optimize.

I think if we could run existing specs (not all) as benchmarks, that would be a good start.

I imagine we could do:

```
bundle exec rake liquid_spec ... --benchmark --run 8 --warmup 5
```

The benchmarking logic should be similar to https://github.com/Shopify/liquid-wasm/blob/main/bench/run.rb.