Make ComputedStyle fields private and add getters.

This isolates layout code from upcoming refactoring in properties.rs.mako.
Un-disable and fix some warnings in the style crate.
Remove CowArc and use the new Arc::make_unique() method instead.
auto merge of #2478 : SimonSapin/servo/style-getters, r=metajack

This isolates layout code from upcoming refactoring in properties.rs.mako.