chore: bump criterion 0.5 -> 0.8

[dobby-coder]

Bumps criterion 0.5 → 0.8 (dev-dep). Replaces deprecated criterion::black_box with std::hint::black_box in benches.

Refs #15.

Why this PR is small

The tracking issue lists nine outdated direct/dev deps. After investigation, only this one is straightforward to land right now. The rest are blocked or risky:

Package	Status	Reason
criterion 0.5 → 0.8	this PR	dev-dep, no API entanglement
bincode 2 → 3	blocked	bincode 3.0.0 on crates.io is an intentional compile_error! tombstone (see bincode-org/bincode#774). The real successor is unstable / under another name. Stay on 2 for now.
rand_core 0.6 → 0.10	blocked	curve25519-dalek 4.x (latest stable: 4.1.3) pins rand_core 0.6 for Scalar::random / OsRng. curve25519-dalek 5.0-pre.6 is the only release wired to rand_core 0.10 and is still pre-release.
rand 0.8 → 0.10 (dev)	blocked	Same root cause: rand 0.10 re-exports rand_core 0.10::RngCore, which doesn't satisfy setup/keygen/sign's RngCore + CryptoRng bound (which still resolves to rand_core 0.6). The doctest and benches all flow thread_rng() into those bounds.
sha3 0.10 → 0.11	blocked	sha3 0.11 needs digest 0.11. Scalar::from_hash in gg.rs:233 and the curve25519-dalek/digest feature are on digest 0.10, so the Sha3_512/Scalar boundary breaks. Lands with the curve25519-dalek 5 bump.
bincode 2 → 3 (dev), rand_xorshift 0.3 → 0.5 (lock), csv 1.1 → 1.4 (lock), sha2 0.9 → 0.11 (lock)	blocked or transitive	All resolve once rand_core 0.10 / digest 0.11 land.

So the realistic plan is: wait for curve25519-dalek 5.0 stable, then do a single cross-cutting bump (rand + rand_core + sha3 + digest + curve25519-dalek 5). I'll re-evaluate when that ships.

Verified locally

• cargo build --all-features
• cargo test --all-features (15 tests + 1 doctest pass)
• cargo bench --no-run --all-features
• cargo fmt --all -- --check
• cargo build --target wasm32-unknown-unknown --all-features --lib (no-std)

Reviewer quickstart

```bash
git fetch origin && git checkout chore/dep-bumps && cargo test --all-features
```