feat(data/finset/basic): Add `coe_pair` lemmas (#16235)
feat(data/finset/basic): Add `coe_pair` lemmas (#16235)

Adds `coe_pair` lemmas to go with `coe_singleton` and `coe_eq_singleton` (and fixes a couple of style issues).





leanprover-community/mathlib,leanprover-community/mathlib