 fix(algebra/ring): add coe_neg_one lemma to units (#3489)
 fix(algebra/ring): add coe_neg_one lemma to units (#3489)

Follow up to #3472 - adds `coe_neg_one`, which allows `norm_cast` to handle hypotheses like `↑-1 = 1`





leanprover-community/mathlib,leanprover-community/mathlib