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/lib.rs

@@ -188,6 +188,7 @@ pub use html5ever::Prefix;
 #[cfg(feature = "servo")]
 pub use servo_atoms::Atom;
 
+pub use style_traits::arc_slice::ArcSlice;
 pub use style_traits::owned_slice::OwnedSlice;
 
 /// The CSS properties supported by the style system.