Terminate to implement the auto loaders infra

[prusse-martin]

The switch from explicit loaders (each description has a loader function) to auto loaders (attributes have metadata that allow a suitable loaders to be used) begun with the introduction of multidata support. But most explicit loaders have been left in-place for the unaffected descriptions.
A full switch will facilitate further development/addition of descriptions.
An example is #210, where the changes to explicit loaders will not be necessary.

• Port all description declarations to populate the metadata (when it is not a plain value like a str, int, or bool), see attrib_scalar, attrib_scalar, attrib_scalar, attrib_scalar, attrib_instance_list, attrib_enum, ... (the ones with "metadata"), Update all Array properties so we can automatically convert those from "alfacase" #199 is related to this;
• Make the explicit loaders progressivelly just return the result of load_instance(TYPE_OF_DESCRIPTION);
• Update alfacase_to_case_helper to not use explicit loaders than remove the explicit loaders (some exist just for the sake of the test there and is no longer used in production);