Generators of arbitrary values

[effectfully]

We generate a lot of arbitrary values for various kinds of things in a multitude of ways. Kenneth did an amazing job at keeping this part of code sane, but I constantly mess with it, so we need to ensure that my shenanigans won't silently ruin the carefully designed code. We also need to automatically test that generators generate interesting enough values of distinct shapes and that coverage is good. Again, Kenneth has a procedure for that, but I think we haven't merged it yet.