Enforce rustfmt in CI

.github/workflows/rustfmt.yaml

@@ -0,0 +1,18 @@
+on:
+  push:
+    branches: [ staging, trying, master ]
+  pull_request:
+
+name: Rustfmt check
+jobs:
+  fmt:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: stable
+          protile: minimal
+          components: rustfmt
+      - name: Check fmt
+        run: cargo fmt -- --check