misc fixes

internal/object/int.go

@@ -65,6 +65,9 @@ func (i Integer) Mul(other Object) (Object, error) {
 func (i Integer) Div(other Object) (Object, error) {
 	switch obj := other.(type) {
 	case Integer:
+		if obj.Value == 0 {
+			return nil, NewDivisionByZeroError(i)
+		}
 		return Integer{i.Value / obj.Value}, nil
 	default:
 		return nil, NewBinaryOpError(i, other, "/")