Improve code cleanliness to avoid future bugs.

Data.CSS.Style.Selector.Index: fix potential crash.

Code cleanliness: Interpret Data.CSS.Style.Importance.splitProperties return value correctly.

Factorize out cascade logic.

Aid attribute sorting, fix install of internal modules.

Test/fix cascade logic for selector specificity.

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

Draft alternate API.