feat(algebra/group_with_zero): add zero_mul_eq_const and mul_zero_eq_const (#11021)
feat(algebra/group_with_zero): add zero_mul_eq_const and mul_zero_eq_const (#11021)

These are to match the corresponding lemmas about unapplied application of multiplication by 1. Like those lemmas, these are not marked simp.





leanprover-community/mathlib,leanprover-community/mathlib