add code coverage (#313)

.github/workflows/rust.yml

@@ -93,3 +93,33 @@ jobs:
           command: clippy
           args: -- -D warnings
         name: Run `cargo clippy`
+
+  coverage:
+    name: Code Coverage
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        name: Checkout
+      
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: nightly
+          override: true
+        name: Install Rust nightly
+      
+      - name: Run cargo-tarpaulin
+        uses: actions-rs/tarpaulin@v0.1
+        with:
+          version: '0.12.3'
+          args: '--ignore-tests -- --test-threads 1'
+
+      - name: Upload to codecov.io
+        uses: codecov/codecov-action@v1
+
+      - name: Archive code coverage results
+        uses: actions/upload-artifact@v1
+        with:
+          name: code-coverage-report
+          path: cobertura.xml
+