Restructure the inspector API

We currently have various inspector in the CAF main namespace:
- `json_reader`
- `json_writer`
- `stringification_inspector`
- ...

To at least get the implementations details out of the public headers, we moved to a PIMPL-based approach. However, this is a one-off in CAF. We generally move towards interfaces in the public API and the factory pattern to avoid having to expose implementation details at all. Implementations should move into the internal or detail namespace (or an anonymous namespace as implementation detail of the factory).

For CAF 2.0, we should get our inspector consistent with the overall design:
- [ ] ~add a new namespace for our inspectors~
- [ ] ~add the interfaces for out inspectors to that namespace~
- [ ] ~provide factory functions to get instances of the inspectors~
- [ ] ~re-implement and deprecate the current set of classes~
- [x] remove the `fast_pimpl` class again