Mini-rust in Rust 013: Fixing a bug in our lexer

We have been putting up with a weird problem with our lexer: up to now it requires white space between each token. But, last time we created a backtracking iterator that we can use to fix this problem, so let's go ahead and do it. More info: https://artificialworlds.net/blog/2025/01/24/mini-rust-in-rust-013-fixing-a-bug-in-our-lexer