`bench-deploy`: Convert between Criterion JSON formats

After #44

Currently, our `gh-pages` benchmark JSONs can't be used with `criterion-table` and our `commit-comment` benchmarks can't be used in plots. After some testing I've found that reformatting the JSON files is fairly straightforward using `jq`. This would have several benefits:
- Run only one benchmark per commit, rather than fiddling with env vars to produce several for use in our various comparison tools. This saves time and UX hassle
- Cached `commit-comment` benchmarks could be immediately deployed when a PR is merged, rather than having to re-run the benchmark in the `gh-pages` format for plotting & deployment. This saves time, runner resources and clutter in `gh-pages`

Trace the existing gh-pages and commit-comment benchmark workflow paths and inspect the Criterion JSON formats they produce. Use jq to define the conversion between those formats, then verify that one benchmark per commit can support criterion-table, plots, and deployment without separate reruns.

Written by the indexing model from the issue text.