TODO: Remove all this bullshit like rng.h, keep pointer, current boxed, and util; keep the error stuff and get a consistent framework for it but then work on the ruleset, rule definitions, making it ergonomic and `constinit`, etc. The actual real reason this project exists...
Then add things like `iterator<T>` to use for parsing, and `argument_{input/output}_stream` for transformations, etc. See notes on phone for custom handlers as well. Use as much aggregate (POD) init as possible, e.g:

```
// with rule ctor for `<size_t DN>(char8_t, const char &(desc)[DN], kind, attribute = {})`
// ...
 { 'v', "Increase verbosity" , kind::Switch, { .allow_multuple = 3 }},
// ...
```