feat(data/set/intervals): more lemmas (#1665)
feat(data/set/intervals): more lemmas (#1665)

* feat(data/set/intervals): more lemmas

* Use `simp` in more proofs, drop two `@[simp]` attrs

* Drop more `@[simp]` attrs

It's not clear which side is simpler.

leanprover-community/mathlib,leanprover-community/mathlib