CI: Add cargo fmt step

.github/workflows/ci.yml

@@ -52,3 +52,18 @@ jobs:
         with:
           token: ${{ secrets.GITHUB_TOKEN }}
           args: --all-features
+
+  lint:
+    name: Rustfmt
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: actions-rs/toolchain@v1
+      with:
+          toolchain: stable
+          override: true
+    - run: rustup component add rustfmt
+    - uses: actions-rs/cargo@v1
+      with:
+        command: fmt
+        args: --all -- --check