Syntax coloring and more compact diagram

[royalstream]

Two cosmetic improvements:

• New content was added a few days ago to the Closures chapter but it was missing rust's syntax coloring.
• Also, in the Crates and Modules chapter, a diagram was improved to be more symmetric and to take less space.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Manishearth (or someone else) soon.

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[steveklabnik]

@bors: r+ rollup

Thank you! I am so sorry this slipped through the cracks.

[bors]

📌 Commit e9f6c0f has been approved by steveklabnik