`bench-deploy`: Convert between Criterion JSON formats

[samuelburnham]

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