feat(data/fintype): finset.univ_map_embedding (#2765)
Adds the lemma
```
lemma finset.univ_map_embedding {α : Type*} [fintype α] (e : α ↪ α) :
(finset.univ).map e = finset.univ :=
```
Co-authored-by: Scott Morrison <scott.morrison@gmail.com>
Co-authored-by: Bryan Gin-ge Chen <bryangingechen@gmail.com>
