StyleSheet: reintroduce "atomic css" and cache expensive work

[necolas]

Mapping styles to class names helps to improve performance and plays nice with existing CSS code/libraries. The previous atomic-css strategy ensures no style collisions and has benefits for SSR - smaller stylesheets, easier client-side hydration. Correctly resolving styles can be expensive so we should cache results as much as possible. Early indications are that common cases may see a 10x performance improvement.