Use euclid data types for physical structs in layout_2020

This removes a bit of duplication and allows layout_2020 to benefit from
a much richer set of utilities.

components/style/values/computed/length.rs

@@ -715,7 +715,7 @@ impl CSSPixelLength {
     }
 }
 
-impl Zero for CSSPixelLength {
+impl num_traits::Zero for CSSPixelLength {
     fn zero() -> Self {
         CSSPixelLength::new(0.)
     }