test: Instructions benchmark (#175)

## What?

Adds an instructions benchmark.

## Why?

Allows to check for performance regression in CI.

Author: slowli

.github/workflows/ci.yml

@@ -11,6 +11,8 @@ env:
   msrv: 1.85.0
   # Nightly Rust toolchain for checking no-std support.
   nightly: nightly-2025-11-02
+  # Explicitly set the regression threshold for `yab` benchmarks
+  CACHEGRIND_REGRESSION_THRESHOLD: 5%
 
 jobs:
   build-msrv:
@@ -42,6 +44,12 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Install valgrind
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y --no-install-suggests --no-install-recommends valgrind
+          valgrind --tool=cachegrind --version
+
       - name: Install Rust
         uses: dtolnay/rust-toolchain@master
         with:
@@ -87,6 +95,9 @@ jobs:
       - name: Run equivalence
         run: cargo run -p elastic-elgamal --features serde --example equivalence
 
+      - name: Benchmark instructions
+        run: cargo bench --bench instructions -- --vs pub:main --verbose --breakdown
+
   build-nostd:
     runs-on: ubuntu-latest