Implement :is, :where, & :not functional pseudoclasses. This is needed by some of the lowerings for other pseudoclasses. TODO correctly compute specificity.
Prepare to interpret functional pseudoclasses.
Improve documentation, release version 1.2
Rework psuedoelement infrastructure so they can be their own boxes.
Add support for psuedoelements/classes.
Code cleanliness fixes.
Draft alternate API.