Add benchmarks

Can use the [Continuous Benchmark](https://github.com/marketplace/actions/continuous-benchmark) GitHub action to track performance over time:
* Using `customSmallerIsBetter` template
* The Haskell `criterion` library for benchmarks already has a `--template json` option
* Just need to write a converter from `foo.json` to the `benchmark.json` format that `customSmallerIsBetter` expects (break out into separate library?)

Start by reading the Continuous Benchmark action's customSmallerIsBetter template and Criterion's --template json output, since the issue names no repository files or tests. Define the conversion boundary and whether it belongs in a separate library; done means producing the benchmark.json format required by the action and wiring it to track performance over time.

Written by the indexing model from the issue text.