mathlib3

a0adcc04 - chore(category_theory/zero): lemmas for has_zero_morphisms.comp_zero|zero_comp (#4142)

chore(category_theory/zero): lemmas for has_zero_morphisms.comp_zero|zero_comp (#4142) The axioms of `has_zero_morphisms` never had lemmas written for them, so everyone has been using the typeclass fields directly. Co-authored-by: Scott Morrison <scott.morrison@gmail.com>