Testround for Gentype v12

[zth]

We should do a somewhat thorough round of testing of Gentype for v12. Get a grip of what the current issues are and which (if any) we should tackle for v12 (and which to tackle after).

[zth]

Shims for stdlib extended and tested in: #7842

Not sure what more to test right now, but leaving open for a while in case something comes up.

[cknitt]

Done, no more known regressions that we will fix in v12.