ci: add benchmarking workflow.

This workflow runs `cargo bench` on rust stable to prevent bitrot of the
benchmarking tests.

.github/workflows/bench.yml

@@ -0,0 +1,15 @@
+name: Benchmarking
+permissions:
+  contents: read
+on:
+  pull_request:
+  push:
+jobs:
+  linting:
+    runs-on: ubuntu-latest
+    name: Bench
+    steps:
+      - name: Check out source repository
+        uses: actions/checkout@v3
+      - uses: dtolnay/rust-toolchain@stable
+      - run: cargo bench