Improve code coverage reports

[fasterthanlime]

The latest cargo-llvmcov release has support for Rust nightly's experimental branch coverage that just dropped:

• Add unstable --branch flag taiki-e/cargo-llvm-cov#356

There's an experimental flag to ignore unused generics/unused functions:

• https://doc.rust-lang.org/rustc/instrument-coverage.html#-c-instrument-coverageoptions