Enforce rustfmt on CI

Fixes #1214

.github/workflows/main.yml

@@ -8,6 +8,21 @@ env:
   RUSTFLAGS: "-Dwarnings" # Deny warnings on CI
 
 jobs:
+  rustfmt:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Install rustfmt
+      run: |
+        rustup component add rustfmt
+
+    - name: Rustfmt
+      run: |
+        cargo fmt --check
+
   build:
     runs-on: ${{ matrix.os }}
     timeout-minutes: 60