Check Cargo.lock freshness on CI (#1565)

Check that `Cargo.lock` is up-to-date on CI so we're not having to push messy lockfile fix ups after releases.

.github/workflows/test-suite.yml

@@ -123,6 +123,8 @@ jobs:
     - uses: actions/checkout@v1
     - name: Lint code for quality and style with Clippy
       run: make lint
+    - name: Certify Cargo.lock freshness
+      run: git diff --exit-code Cargo.lock
   arbitrary-check:
     name: arbitrary-check
     runs-on: ubuntu-latest