Standard library unsoundness found by Claude Mythos

The Rust Foundation recently received access to Claude Mythos, and has been using it to review the standard library for security issues. The more severe issues are being kept under embargo for now, but a couple minor ones are now public:

• Potential out-of-bounds write upon allocation failure in CString::clone_into(): https://github.com/rust-lang/rust/pull/155707
• Heap overflow in slice::join() with certain pathological Borrow impls: https://github.com/rust-lang/rust/pull/155708

Thanks to the Rust Foundation, wg-security-response, and Anthropic for working to find and fix these issues!