I know this is still a draft, but it's looking pretty good!

However, this needs updating the tests. Also, I'm seeing a lot of code changes, but I don't think the new information is bubbled up in any error messages yet?

@jmmv Yup, gonna add tests when I next get the chance to work on this. Currently nothing actually makes use of the span information yet, correct -- I was done for the day when writing and knew that I would take forever to add error messages :) (not to mention all the self-bikeshedding I'd do)

Thanks for your contribution. I took inspiration in your first change and started doing this... but adding line and column numbers to errors turned out to be a gargantuan task. Adding these for parser errors was easy, but propagating the details for runtime errors was a very different story. In any case, this is now coming up in #191 and should be ready for merge soon.