Protecting the Rust standard library from accidental breakage

Accidental breakage can happen in any codebase. The Rust standard library isn't magically exempt from this — so it too now uses cargo-semver-checks to prevent accidental breakage. Here's why this took months of work by multiple Rustaceans, dozens of pull requests, and 15,000+ lines of code across the Rust repo, cargo-semver-checks, and its component libraries.