mathlib3

096bdb7a - refactor(group_theory/solvable): move subgroup commutators into new file (#8534)

refactor(group_theory/solvable): move subgroup commutators into new file (#8534) The theory of nilpotent subgroups also needs a theory of general commutators (if H,K : subgroup G then so is [H,K]), but I don't really want to import solvable groups to get nilpotent groups, so I am breaking the solvable group file into two, splitting off the API for these commutators.