ci: add formatting checker

.github/workflows/ci.yml

@@ -18,3 +18,14 @@ jobs:
 
     - name: Run tests
       run: cargo test --verbose
+
+  rustfmt:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+
+    - name: Check formatting
+      run: |
+        cargo fmt -- --check