Mini-rust in Rust 040: While conditions should be bool

Working up towards some difficult type-checking, we do some easy type-checking: the condition part of a while condition should be a boolean, and to get there we need to improve the type we give to the result of an operation. More info: https://artificialworlds.net/blog/2025/08/01/mini-rust-in-rust-040-while-condition-should-be-bool