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_traits/lib.rs

@@ -84,6 +84,7 @@ pub enum CSSPixel {}
 //   / hidpi_ratio => DeviceIndependentPixel
 //     / desktop_zoom => CSSPixel
 
+pub mod arc_slice;
 pub mod specified_value_info;
 #[macro_use]
 pub mod values;