add rustfmt check

.github/workflows/rustfmt.yaml

@@ -0,0 +1,24 @@
+
+on:
+  push:
+    branches: [ staging, trying, master ]
+  pull_request:
+
+name: Code formatting check
+
+jobs:
+  fmt:
+    name: Rustfmt
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+          components: rustfmt
+      - uses: actions-rs/cargo@v1
+        with:
+          command: fmt
+          args: --all -- --check