Mini-rust in Rust 030: Expressions after while loops

It turns out we are making a bad assumption in our parser: that every statement ends with a semicolon. That's not quite right - for example, while loops can form statements, but they don't need to be ended with a semi-colon. Let's fix the bug! More info: https://artificialworlds.net/blog/2025/05/25/mini-rust-in-rust-030-expression-after-while