Switch from grcov to cargo-llvm-cov

[elasticdog]

This tool provides a much better workflow compared to what we were doing with our grcov-based xtask. It's well-written, popular, and receives regular maintenance, so it should be easier to keep on top of upstream changes. It will also be trivial to add publishing to codecov.

See:

• https://github.com/taiki-e/cargo-llvm-cov

Checklist

• Ran cargo xtask fixup for project formatting and style
• Modified all relevant documentation
• Updated CHANGELOG.md per "Keep a Changelog" format
• Added tests covering my changes

[elasticdog]

Note that I'm no longer explicitly installing the toolchain component llvm-tools-preview or mentioning it in the docs, even though it is still a requirement. The cargo-llvm-cov tool handles it in a nicer fashion, taking into account whether or not the tool is being run on CI, and I'd rather the upstream project manage its own dependency directly.

See:

• Improve handling of cases where llvm-tools-preview is not installed taiki-e/cargo-llvm-cov#219