Remove Stylist-Traits APIs from Stylist Core, leaving backwards-compatible stubs.

Start testing counters implementations, fix syntactic shorthand lowering for counter[s]() functions.

Evaluate attribute namespace selectors.

Optimize: use HashMap.Lazy for cascade.

Improve documentation, release version 1.2

Write reference documentation.

Test rules capturing of ConditionalStyleSheet.

Code cleanliness: drop unneeded ' from Data.CSS.Style.Cascade.dispatch'

Test CSS variables.

Implement var() via a PropertyParser decorator.

Expose hooks for implementing var().

The tricky part here is around CSS inheritance, it works naturally in the existing cascade implementation.

Resolve attr() functions.

It's defined during query, because that's when Stylish Haskell (or it's callers)
have easy access to the element being queried.

Rework psuedoelement infrastructure so they can be their own boxes.

Code cleanliness fixes.

Factorize out cascade logic.