mathlib3

d821860f - feat(group_theory/group_action/basic): class formula, Burnside's lemma (#8801)

feat(group_theory/group_action/basic): class formula, Burnside's lemma (#8801) This adds class formula and Burnside's lemma for group action, both as an equiv and using cardinals. I also added a cardinal version of the Orbit-stabilizer theorem. Co-authored-by: Patrick Massot <patrickmassot@free.fr>