- Refactor using common types that were once internal
- Removes ancient macro approaches to C++20 solutions
- Changes `Exception` message type to use std::string
* std::string_view isn't worthwhile in this context
- Factor out respective common from macro and apply to all scopes
* `dfi::common`
* `dfi::macro::common`
* `dfi::plugin::common`
- Add deprecation warnings in `dfi::macro::common`
* Common functionality (not macro specific) is now in `dfi::common`
- Add convenience wrappers to inner common impl, where appropriate
* Allows for backwards compatibility
- Add/update/clarify code docs and Doxygen definitions
- Related refactoring
NOTE: all changes are backward compatible (no need to bump major).
