style: Introduce ArcSlice, a small wrapper over ThinArc but without an explicit header.

We could make the header PhantomData or something, but then we wouldn't be able
to bind to C++, since C++ doesn't have ZSTs. So add a canary instead to add a
runtime check of stuff being sane.

Differential Revision: https://phabricator.services.mozilla.com/D30133

components/style/values/animated/mod.rs

@@ -462,3 +462,17 @@ where
         Ok(v.into_boxed_slice())
     }
 }
+
+impl<T> ToAnimatedZero for crate::ArcSlice<T>
+where
+    T: ToAnimatedZero,
+{
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        let v = self
+            .iter()
+            .map(|v| v.to_animated_zero())
+            .collect::<Result<Vec<_>, _>>()?;
+        Ok(crate::ArcSlice::from_iter(v.into_iter()))
+    }
+}