Refactor style to be completely backend-independent

This commit refactors the style crate to be completely independent of
the actual implementation and pseudo-elements supported.

This also adds a gecko backend which introduces parsing for the
anonymous box pseudo-elements[1], although there's still no way of
querying them.

https://mxr.mozilla.org/mozilla-central/source/layout/style/nsCSSAnonBoxList.h

components/style/lib.rs

@@ -62,6 +62,7 @@ pub mod restyle_hints;
 pub mod selector_impl;
 pub mod selector_matching;
 pub mod sequential;
+pub mod servo;
 pub mod stylesheets;
 pub mod traversal;
 #[macro_use]