mathlib3

15fcb8a1 - feat(algebra/lie_algebra): define equivalences, direct sums of Lie algebras (#2404)

feat(algebra/lie_algebra): define equivalences, direct sums of Lie algebras (#2404) This pull request does two things: 1. Defines equivalences of Lie algebras (and proves that these do indeed form an equivalence relation) 2. Defines direct sums of Lie algebras The intention is to knock another chip off https://github.com/leanprover-community/mathlib/issues/1093