Adding Practice Exercises

With all the effort going (quite rightly!) into the Learning Syllabus, we haven't done much with Practice exercises in the last year or two. In a week when I can't summon the mental energy for anything more demanding, I've been poking at a few of the 53-exercise backlog.

Current status for Julia and a few other tracks is on [Sheets](https://docs.google.com/spreadsheets/d/1eHVKuQ-5QnAr0DLoF2-lZNE7CxSe30Cbk85os6qz2OM/edit?usp=sharing). This is current for mid-September, but won't automatically update. I find it useful to locate tracks which have implemented some of the newer exercises. It's also interesting to see which exercises have been marked "foregone" by multiple tracks, suggesting they are maybe not our top priority.

Are there any missing exercises which are a poor fit for Julia (or a personal bugbear for the maintainers)? If so, we should add them to a `foregone` list in `config.json` to take them out of consideration.