Mini-rust in Rust 031: Getting "while" through the type checker

Our while loop now parses, but it doesn't make its way through the type-checker unscathed. We're not quite ready to do real type checking here, but we need to pass this code through so we can actually run it! More info: https://artificialworlds.net/blog/2025/05/30/mini-rust-in-rust-031-type-checking-while