Rust 1.55.0

# `unsafe`

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

> Rust has a second language hiding out inside of it, unsafe Rust, which does not enforce memory safety guarantees.

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

- in the current edition: Ch 19.01 — Unsafe Rust
- The Rustonomicon, The Dark Arts of Advanced and Unsafe Rust Programming
- In the first edition: Ch 3.36 — `unsafe`