TODO

* Another Opt structure with -v and such.
* Run the backgroud thread as a task in tokio?
* Why C needs to be Debug?
* config_exts_all?

* unify Order of Opts/Config

* Reintroduce scaling
* Parts of the core spirit can be split off and useful separately ‒ base traits, config loading. Then, if the user doesn't want the background thread and stuff, it can be used manually.
* Being able to configure the tokio thread pool
  - As a fragment
  - Have a callback/time when we already have the config loaded byt we can still create the singleton and body wrapper ‒ like, on_build?
* Make some of the traits sealed
* Builder::run should not need the closure to be static.

* Stackable and optional for references, eg Vec<&Fragment> or Option<&Fragment>
* Use humantime-serde to get rid of our own humantime/serde integration