mathlib3

24ed74a2 - feat(algebra/category/Semigroup/basic): categories of magmas and semigroups (#6387)

feat(algebra/category/Semigroup/basic): categories of magmas and semigroups (#6387) This PR introduces the category of magmas and the category of semigroups, together with their additive versions. Co-authored-by: Julian-Kuelshammer <68201724+Julian-Kuelshammer@users.noreply.github.com> Co-authored-by: Scott Morrison <scott.morrison@gmail.com>