feat(group_theory/complement): API lemmas relating `range_mem_transversals` and `to_equiv` (#13694)
feat(group_theory/complement): API lemmas relating `range_mem_transversals` and `to_equiv` (#13694)

This PR adds an API lemma relating `range_mem_left_transversals` (the main way of constructing left transversals) and `mem_left_transversals.to_equiv` (one of the main constructions from left transversals), and a similar lemma relating the right versions.





leanprover-community/mathlib,leanprover-community/mathlib