feat(set_theory/ordinal/basic): tweak `type_def` + golf `type_lt` (#14611)
feat(set_theory/ordinal/basic): tweak `type_def` + golf `type_lt` (#14611)

We replace the original, redundant `type_def'` with a new more general lemma. We keep `type_def` as it enables `dsimp`, unlike `type_def'`. We golf `type_lt` using this new lemma.





leanprover-community/mathlib,leanprover-community/mathlib