chore: also run benchmarks on CI

The bencharks are run on CircleCI, also run them on GitHub Actions.

.github/workflows/ci.yml

@@ -166,3 +166,10 @@ jobs:
 #
 #    - name: Docs
 #      run: cargo doc
+
+  # Benchmarks need a nightly Rust
+  bench:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Run cargo bench
+      run: cargo +nightly bench