Generate Markdown summary report for GitHub Actions CI

[karlmdavis]

I was taking a stab tonight at adding benchmarking runs to my CI (I know the data quality would be lousy due to the oversubscribed & virtualized environment, but it's still useful enough for my use cases).

Unfortunately, the https://github.com/actions-rs/cargo step doesn't support capturing console output, so I can't just grab stdout from that and shove it into a comment. (And even if I could, that'd have a lot of compilation, etc. noise.)

As an alternative, it'd be great if cargo criterion produced a single, summary Markdown report file in target/criterion/reports. I could just grab that file's contents, shove it into a GitHub comment, and bingo: a great benchmarking UX as part of CI for my projects.

[brainstorm]

There's this github action called github-action-benchmark that does support output extraction and there's a PR about to support this: benchmark-action/github-action-benchmark#138

Not exactly like serializing a Markdown report file, but that'd be the next best thing I guess?

Plus I'm working on cargo-criterion support for it: benchmark-action/github-action-benchmark#145