Publish to crates.io

The crates/publish folder contains a placeholder crate, so that the book could be located when searching on crates.io⮳.

• cargo update if necessary.
• Go to crates.io⮳, sign in, and create an API token in Account Settings > API Tokens.
• Use cargo login to save the token in $CARGO_HOME/credentials.toml.
• cd crates/publish.
• cargo build --locked --release.
• cargo clippy.
• cargo run --release.
• cargo doc.
• Review cargo package --list.
• cargo package.
• Review the packaging output in /code/target/bk/package.
• When ready, cargo publish --dry-run; cargo publish.