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. You can find the source code at codeberg.org&#x2F;andybalaam&#x2F;milk and more of my stuff at artificialworlds.net