mathlib

2d395a99 - refactor(algebra/pi_instance): delete pi_instance file, and move instances to group/ring etc appropriately (#3513)

refactor(algebra/pi_instance): delete pi_instance file, and move instances to group/ring etc appropriately (#3513) Co-authored-by: Bryan Gin-ge Chen <bryangingechen@gmail.com> Co-authored-by: Chris Hughes <chrishughes24@gmail.com>