Rust 1.37.0

# Testing

There is a new edition of the book and this is an old link.

> Rust includes support for writing software tests within the language itself.

    #[cfg(test)]
    mod tests {
        #[test]
        fn it_works() {
        }
    }Run

Here are the relevant sections in the new and old books:

- in the current edition: Ch 11.00 — Testing
- In the first edition: Ch 4.2 — Testing