style: Move all the style sharing code outside matching.rs

This is just a code health change. I want to move it away to keep matching.rs as
simple as possible.

components/style/lib.rs

@@ -121,6 +121,7 @@ pub mod rule_tree;
 pub mod scoped_tls;
 pub mod selector_parser;
 pub mod shared_lock;
+pub mod sharing;
 pub mod stylist;
 #[cfg(feature = "servo")] #[allow(unsafe_code)] pub mod servo;
 pub mod sequential;