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• Make sure cargo outdated -R is clean in the root and in b3sum/.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 -h output in b3sum/README.md if it's changed.
• Bump BLAKE3_VERSION_STRING in c/blake3.h.
• Bump VERSION in c/CMakeLists.txt.
• 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/.