add a cargo-fmt job to CI

.github/workflows/checks.yml

@@ -13,7 +13,7 @@ jobs:
     name: "Clippy"
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
 
       - name: Cache cargo registry
         uses: actions/cache@v2
@@ -51,7 +51,7 @@ jobs:
     name: "Documentation"
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
 
       - name: Setup mdBook
         uses: peaceiris/actions-mdbook@v1
@@ -81,3 +81,22 @@ jobs:
 
       - run: mdbook test docs
       - run: mdbook build docs
+
+  fmt:
+    runs-on: ubuntu-latest
+    name: "Formatting"
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          components: rustfmt
+          toolchain: stable
+          override: true
+
+      - uses: actions-rs/cargo@v1
+        with:
+          command: fmt
+          args: --all -- --check
+