Exercise Ordering

I'm not sure if the exercise ordering makes much sense. The core exercises and the exercise ordering was setup in #187, and they may need to be updated or changed. (I know of at least a couple changes I want to see happen, which I'll list here).

We're looking for someone to take on this task, figuring out if the ordering makes sense now after it's been active for several months, and possibly reordering many of the exercises. This would work best if we got feedback from the mentors for the track, and possibly from current or recent students.

I can help and provide input, but I may not have time to take this on myself. And I haven't mentored or done any exercises on exercism.io since they launched the new version of the website, so I don't personally have great context for what would make sense.

The one thing I want to happen is to make Gigasecond and Meetup not part of the core track to help with #233. I may end up making just that change myself if necessary, but the rest of the exercises need to be looked at as well.

This visualizer may help: https://github.com/jonmcalder/exercism-config-viz

**Important Note:** Any changes to the core exercises will need some manual work elsewhere to update the website. See #262 for an example.