Rename crate and binary to cargo-slow

[LucaCappelletti94]

Renames slow-rs to cargo-slow so it installs as a cargo subcommand and runs as cargo slow. Updates Cargo.toml (name, bin, repository/homepage/docs.rs URLs), the README install and usage, the release workflow, badges, doc comments, and UI strings. main.rs drops the slow token cargo injects for cargo slow ..., so both that and a direct cargo-slow call parse the same. fmt, clippy, and all 57 tests pass.