mathlib3

cb2b02ff - feat(representation_theory/basic): representation theory without scalar actions (#13573)

feat(representation_theory/basic): representation theory without scalar actions (#13573) This PR rewrites the files `representation_theory/basic` and `representation_theory/invariants` so that they avoid making use of scalar actions. It also includes the new definitions and lemmas of PR #13502 written with this new approach. Co-authored-by: antoinelab01 <66086247+antoinelab01@users.noreply.github.com>