mathlib

0d052998 - chore(data/finset): rename `ext`/`ext'`/`ext_iff` (#3069)

chore(data/finset): rename `ext`/`ext'`/`ext_iff` (#3069) Now * `ext` is the `@[ext]` lemma; * `ext_iff` is the lemma `s₁ = s₂ ↔ ∀ a, a ∈ s₁ ↔ a ∈ s₂`. Also add 2 `norm_cast` attributes and a lemma `ssubset_iff_of_subset`.