Add additional hooks for callers:

* Resolve relative URLs.
* Set priority on any stylesheet implementation.
* Substitute content during XML Conduit traversal in place of computed styles.

Rework psuedoelement infrastructure so they can be their own boxes.

Code cleanliness fixes.

Factorize out cascade logic.

Add method to override CSS inheritance.

Test cascade logic.

Test/fix cascade logic for selector specificity.

Switch to new namespace for consistency with other modules like css-syntax.

Draft alternate API.