Feature: Improve add-on init dispatch

Handle different add-on `init()` signatures in a generic and type safe way. An adapter is selected at compile-time based on the signature of the init function. The adapter is stored in the module struct. At run-time the adapter unpacks the users init function and provides the requested arguments.

This is related to #8802.
Just harmonizing the backslashes in the big macro block... like it was before the change. I liked it so I restored it. Please take a look at the diff in split view or look at the raw file.