style: Add a Zero trait that doesn't require Add, and use it in place of num_traits and IsZeroLength.

Use it to be consistent in InsetRect serialization and storage between Servo and
Gecko.

Differential Revision: https://phabricator.services.mozilla.com/D21493

components/style/properties/helpers.mako.rs

@@ -19,6 +19,8 @@
         #[allow(unused_imports)]
         use crate::values::specified::AllowQuirks;
         #[allow(unused_imports)]
+        use crate::Zero;
+        #[allow(unused_imports)]
         use smallvec::SmallVec;
         pub use crate::values::specified::${type} as SpecifiedValue;
         pub mod computed_value {