deps: bump criterion 0.5 -> 0.8 and switch to std::hint::black_box

Dependabot opened #10 to bump criterion from 0.5.1 to 0.8.2 but the
straight version bump fails to build: criterion::black_box was
deprecated in 0.6 and removed/marked deny-warn in 0.8, so the existing
`use criterion::{black_box, ...}` produces

    error: use of deprecated function `criterion::black_box`: use
    `std::hint::black_box()` instead

across every bench callsite. Switch the import to std::hint::black_box
(stable since Rust 1.66, well under our MSRV of 1.85) and drop the
criterion re-export.

This supersedes #10 — same target version, plus the code change
Dependabot can't make on its own.

Verified locally:
  cargo bench -p wickra --no-run  # builds clean
  cargo clippy --workspace --all-targets --all-features -- -D warnings
  cargo test --workspace  # 630 passed / 0 failed

Cargo.toml

@@ -32,7 +32,7 @@ rayon = "1.10"
 # Testing
 proptest = "1.5"
 approx = "0.5"
-criterion = { version = "0.5", features = ["html_reports"] }
+criterion = { version = "0.8", features = ["html_reports"] }
 
 # Python binding
 pyo3 = { version = "0.28", features = ["extension-module", "abi3-py39"] }