xtask: Add action to generate a code coverage report

[nicholasbishop]

This uses llvm-cov (https://github.com/taiki-e/cargo-llvm-cov) to create a simple HTML code coverage report.

For now this only includes coverage from host tests, but in the future we can use https://github.com/Amanieu/minicov to include coverage from VM tests as well.

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)

[phip1611]

LGTM! Do we want to add an CI step and upload the html file as job artifact?

[nicholasbishop]

I was thinking of using one of the free-for-open-source platforms such as codecov.io or coveralls.io. I've used both, for example https://app.codecov.io/gh/nicholasbishop/ext4-view-rs and https://coveralls.io/github/google/sbat-rs.

I've occasionally looked at doing some within-github-actions implementation that just stores the HTML files, but I've never found a particulary straightforward and easy way to do that, whereas uploading to one of the above websites is quite simple.

[codecov-commenter]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️

[nicholasbishop]

Added a couple commits: add a --lcov switch to generate a raw report, and add a CI job to upload to code coverage (see for example https://app.codecov.io/github/rust-osdev/uefi-rs/pull/1423/blob/uefi/src/data_types/chars.rs#L64)