Make AnimationValue have the same variants as PropertyDeclaration

By making AnimationValue have the same representation as PropertyDeclaration
and Void variants for non-animatable properties, we know by constructions
that all properties have the same discriminant in both.

components/style/lib.rs

@@ -78,6 +78,7 @@ extern crate uluru;
 extern crate unicode_bidi;
 #[allow(unused_extern_crates)]
 extern crate unicode_segmentation;
+extern crate void;
 
 #[macro_use]
 mod macros;