Add code coverage to CI

[zachschuermann]

We can leverage LLVM instrumentation-based coverage information via cargo-llvm-cov for code coverage info. The latter has instructions to run locally - this PR introduces a new CI task called coverage to install/run cargo-llvm-cov and upload results to codecov.

based on top of #284

[nicklan]

looks fine. re-request once we get the token sorted so we can just make sure it does the right thing

[codecov]

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 ☂️

[zachschuermann]

okay looks like its (mostly?) working now. have a link to coverage for the last commit here (but nothing to compare against I guess): https://app.codecov.io/github/delta-incubator/delta-kernel-rs/commit/4fe559d2b07563e88b3e8404fde3218f6f83ce08