Rust 1.44.0

# Concurrency

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

> Historically, programming [concurrency] has been difficult and error prone: Rust hopes to change that. Fearless concurrency allows you to write code that’s free of subtle bugs and is easy to refactor without introducing new bugs.

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

- in the current edition: Ch 16.00 — Fearless Concurrency
- In the first edition: Ch 4.6 — Concurrency