Restore coverage data in CI

This seems to have been accidentally removed in
fa20bd9d44.

.github/workflows/coverage.yml

@@ -0,0 +1,22 @@
+name: Coverage
+
+on: [pull_request, push]
+
+jobs:
+  coverage:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Install Rust
+        run: rustup toolchain install stable --component llvm-tools-preview
+      - name: Install cargo-llvm-cov
+        uses: taiki-e/install-action@cargo-llvm-cov
+
+      - name: Generate code coverage
+        run: cargo +stable llvm-cov --all-features --workspace --lcov --output-path lcov.info
+      - name: Upload coverage to Codecov
+        uses: codecov/codecov-action@v3
+        with:
+          files: lcov.info
+          fail_ci_if_error: false