Investigate use of ZippyJSON in StreamingArchiveDecoder
https://github.com/michaeleisel/ZippyJSON
For large ZTS files (~30GB), we're currently taking about an hour to extract a subreddit. Initial poking suggests that about 2/3 of that time is used to parse JSON in each file, and so there are some substantial savings in speeding up JSON parsing. It's a drop-in replacement for Swift's JSONDecoder(), and so easy to test, as well.