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/values/specified/angle.rs

@@ -101,6 +101,11 @@ impl Angle {
         }
     }
 
+    /// Return `0deg`.
+    pub fn zero() -> Self {
+        Self::from_degrees(0.0, false)
+    }
+
     /// Returns the value of the angle in degrees, mostly for `calc()`.
     #[inline]
     pub fn degrees(&self) -> CSSFloat {
@@ -113,12 +118,6 @@ impl Angle {
         self.was_calc
     }
 
-    /// Returns `0deg`.
-    #[inline]
-    pub fn zero() -> Self {
-        Self::from_degrees(0.0, false)
-    }
-
     /// Returns an `Angle` parsed from a `calc()` expression.
     pub fn from_calc(degrees: CSSFloat) -> Self {
         Angle {