Check rustfmt in CI

Author: tgross35

.github/workflows/rust.yml

@@ -50,10 +50,23 @@ jobs:
 
     - run: cargo build
 
+  rustfmt:
+    name: Rustfmt
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: Install Rust
+      run: |
+        rustup default nightly
+        rustup update --no-self-update
+        rustup component add rustfmt
+    - run: cargo fmt -- --check
+
   success:
     needs:
       - test
       - msrv
+      - rustfmt
     runs-on: ubuntu-latest
     # GitHub branch protection is exceedingly silly and treats "jobs skipped because a dependency
     # failed" as success. So we have to do some contortions to ensure the job fails if any of its