Mini-rust in Rust 039: Spans should be ranges

We've been tracking the location of a token in the file using a pair of numbers, but Rust has a better way: a `Range`. After some debate and wandering, we make the code a little better with this change. The interlude music is [Blood, Collateral, & Otherwise](https://freemusicarchive.org/music/hunk-golden/single/blood-collateral-otherwise/) by [Hunk Golden](https://freemusicarchive.org/music/hunk-golden/), used under the [Attribution 4.0 International License](https://creativecommons.org/licenses/by/4.0). More info: https://artificialworlds.net/blog/2025/07/25/mini-rust-in-rust-039-span-as-range