Sorted out item 7 (module state). Item 8 is mostly in place, although I get ref. count errors if I add traverse/clear callbacks, so I've temporarily disabled those.

Proof-of-concept of multi-phase init can be found here: https://github.com/erlend-aasland/cpython/commits/bpo-42064/all
I'll try to break items 3 and 7 up in smaller pieces for the reviewer's convenience.