Coverage: grcov and Tarpaulin

[Xophmeister]

When code coverage was initially investigated (see #80), it relied on grcov and some LLVM rustup components to make it work. This didn't work well with Nix.

In the meantime, grcov appears to be in nixpkgs, which presumably removes the dependency on rustup to make it work.

Also, during the investigation of #80, Tarpaulin was an alternative tool for code coverage. This has probably matured in the intervening 2+ years and may be a better option.