Use llvm-cov for coverage reporting

grcov reports incorrect coverage, e.g. showing blank lines as uncovered

coverage.sh

@@ -8,7 +8,10 @@ export LLVM_PROFILE_FILE="target/coverage/prof/opentally-%p-%m.profraw"
 export CARGO_TARGET_DIR=target/coverage
 cargo test
 
-grcov target/coverage/prof --binary-path target/coverage/debug -t html -o target/coverage/html \
-	--ignore src/numbers/rational_num.rs \
-	--ignore src/stv/wasm.rs \
-	--ignore 'tests/*'
+llvm-profdata merge -sparse target/coverage/prof/*.profraw -o target/coverage/opentally.profdata
+
+llvm-cov show target/coverage/debug/opentally -instr-profile=target/coverage/opentally.profdata -Xdemangler="$HOME/.cargo/bin/rustfilt" \
+	-ignore-filename-regex="$HOME/." \
+	-ignore-filename-regex=numbers/rational_num.rs \
+	-ignore-filename-regex=stv/wasm.rs \
+	-format=html --show-instantiations=false --output-dir=target/coverage/html