* Move unordered_printers.hpp into 'detail', where it should have been from the beginning
This is not a public header. It is only meant to be consumed as a transitive include.
* Store self.table instead of self.val
* Fix pretty-printers not working when constexpr variables are optimized away
Also do a few other things.
* `groups` and `elements` are now stored inside `self`
* `mask`, `n0`, and `n` aren't declared ahead of their first assignment
* Remove trailing whitespace
* Update the generated pretty-printer header
