Mini-rust in Rust 018: Type checking, the happy path

This is it: we're going to do some actual type-checking. We want to be able to identify that `let x: i32 = 3.0;` is wrong, but for now we're going to check some things that are right. Type-checking this kind of thing is way easier than the full type inference we will eventually need, so it's a gentle start. More info: https://artificialworlds.net/blog/2025/02/28/mini-rust-in-rust-018-type-check-assigment-happy