Add example #12.4, module split files.
Add example #13.1, crate libraries.
Add example #13.2, crate externs.
Add example #14.1, attribute crates
Add example #14.2, attributes cfg.
Add example #14.2.1, custom attributes.
Add example #15, tuples.
Add example #16.0, pattern matching.
Add example #16.1, match destructuring.
Add example #17.0, structures.
Add example #17.1, structure visibility.
Add example #18, generics.