Add workflow to check format in CI

.github/workflows/format.yml

@@ -0,0 +1,12 @@
+name: Format
+on: [push, pull_request]
+jobs:
+  native:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: hecrj/setup-rust-action@v1
+      with:
+        components: rustfmt
+    - uses: actions/checkout@master
+    - name: Check format
+      run: cargo fmt --all -- --check