Getting around (the lack of) covariance to write a generic collection

[DevAndArtist (Adrian Meister)]

And the most recent thread that we really need to push forward:

Generalized supertype constraints

This topic is not new, it was previously pitched by @anandabits here and there is also a never merged documentation PR for this idea: As ABI is finally stable it was a fresh breeze to see new generics related pitches arise in SE. I would love if we as the community could push together this topic (if it simplifies things I would start only the supertype constraint and separate the generic associated type (see the PR) out into it's own pitch/proposal). To kick off the discussion I want you to …