feat(data/set/prod): add theorems about `λ x, (x, x)` (#15604)
feat(data/set/prod): add theorems about `λ x, (x, x)` (#15604)

From the sphere eversion project. Also swap LHS with RHS in
`set.diagonal_eq_range` and rename it to `set.range_diag`.

Co-authored-by: Oliver Nash <a5f9e832049bc15c5c5569212bf5fd8572cec019@gmail.com>





leanprover-community/mathlib,leanprover-community/mathlib