Release checklist

Make sure cargo outdated -R is clean in the root and in b3sum/.
Bump the version in the root Cargo.toml.
Bump the version in b3sum/Cargo.toml.
Delete b3sum/Cargo.lock and recreate it with cargo build or similar.
Update the --help output (including the version number) in b3sum/README.md.
Bump BLAKE3_VERSION_STRING in c/blake3.h.
Make a version bump commit with change notes.
git push and make sure CI is green.
git tag the version bump commit with the new version number.
git push --tags
cargo publish in the root.
cargo publish --dry-run in b3sum/ and make sure it fetches the just-published library version.
cargo publish in b3sum/.