Run rustfmt on CI (#178)

* Run rustfmt on CI

* Run rustfmt on nightly toolchain, outside test matrix

Rustfmt doesn't need dependencies and should be platform-agnostic anyway.

.github/workflows/rust.yml

@@ -57,3 +57,19 @@ jobs:
       with:
         command: clippy
         args: --all-targets -- -D warnings
+
+  fmt:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: actions-rs/toolchain@v1
+      with:
+        profile: minimal
+        toolchain: nightly
+        override: true
+        components: rustfmt
+    - name: Rustfmt
+      uses: actions-rs/cargo@v1
+      with:
+        command: fmt
+        args: --all -- --check