Add Zify BinOp Op_div.

Lemma quots_spec (x y : Z) :
((x = -4611686018427387904 /\ y = -1 /\ quots x y = -4611686018427387904)
\/ ((x <> -4611686018427387904 \/ y <> -1) /\ quots x y = Z.quot x y))%Z.