Mini-rust in Rust 008: Refactoring our parser

Our parser kinda works, but it needs some work to be less terrible. We've realised we did a few things in awkward or wrong ways, so let's fix them before they get out of hand. More info: https://artificialworlds.net/blog/2024/12/20/mini-rust-in-rust-008-refactoring-our-parser/