Mini-rust in Rust 021: Including type information in our syntax tree

We realised last time that our type checker needs to move on from just saying "yes" or "no" for whether our program is OK: we need to keep hold of the type information we work out, so we can use it for error messages and possibly even within the evaluator. More info: https://artificialworlds.net/blog/2025/03/21/mini-rust-in-rust-021-typed-syntax-tree