Run formatter in CI

Add a job to the CI pipeline to run `cargo fmt --check` with the nightly
toolchain.

Author: tcharding

.github/workflows/rust.yml

@@ -104,3 +104,19 @@ jobs:
         with:
           command: clippy
           args: --all-targets -- -D warnings
+
+  Fmt:
+    name: Fmt
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: nightly
+          override: true
+      - run: rustup component add rustfmt
+      - uses: actions-rs/cargo@v1
+        with:
+          command: fmt
+          args: --check