refactor(*): kill int multiplication diamonds (#7255)
Insert a data field `gsmul` in `add_comm_group` containing a Z-action, and use it to make sure that all diamonds related to `Z` -actions disappear.
Followup to #7084
Co-authored-by: Scott Morrison <scott.morrison@gmail.com>
Co-authored-by: Eric Wieser <wieser.eric@gmail.com>
Co-authored-by: Mario Carneiro <di.gama@gmail.com>
