Mini-rust in Rust 015: Parsing a typed assignment

We want to have a go at some type-checking, so the first step is to write some code that causes a type error. We are going for `let x: i32 = 3.0;`. To do that, we need to be able to lex and parse the `: i32` part... More info: https://artificialworlds.net/blog/2025/02/14/mini-rust-in-rust-015-parse-typed-assignment