Add CI and gate on cargo fmt output

.github/workflows/ci.yml

@@ -0,0 +1,23 @@
+name: CI
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+jobs:
+  fmt:
+    name: Check formatting
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: actions-rs/toolchain@v1
+      with:
+          toolchain: nightly
+          components: rustfmt
+          override: true
+    - name: Check formatting
+      uses: actions-rs/cargo@v1
+      with:
+        command: fmt
+        args: -- --check