Add upload of code coverage results

Author: sunsided

.github/workflows/rust.yml

@@ -91,6 +91,12 @@ jobs:
         uses: taiki-e/install-action@nextest
       - name: Generate code coverage
         run: cargo llvm-cov nextest --all-features --lcov --output-path lcov.info
+      - name: Upload coverage to Codecov
+        uses: codecov/codecov-action@v4.0.1
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}
+          files: lcov.info
+          fail_ci_if_error: true
 
   build-others:
     name: Build on ${{ matrix.os }}