Write chapter on macros

As discussed briefly in https://github.com/google/comprehensive-rust/pull/130#issuecomment-1486800993, we should have a morning or afternoon section about macros. This would be an optional advanced topic which we will teach to people who are interested after completing the main part of the course.

Inspiration:
- [The Rust Programming Language: Macros](https://doc.rust-lang.org/book/ch19-06-macros.html)
- [Rust by Example: `macro_rules!`](https://doc.rust-lang.org/rust-by-example/macros.html)
- [MacroKata](https://tfpk.github.io/macrokata/)