Cargo and testing

Cargo is Rust's build tool, package manager, test runner, and project skeleton. This theme teaches Cargo.toml, crates, binaries vs libraries, unit tests, integration tests, and assert_eq!. Planned exercises: 1. choose the purpose of Cargo.toml. 2. recognise crate roots. 3. choose #[test]. 4. fill assert_eq!. 5. fill a module test path. 6. write a unit test. 7. write a result-returning test. 8. translate a TS test into Rust assertions. 9. organise a tiny library and binary split.