Expose common interface above all selector passes.

Compute selector specificity.

Reorganize modules to make space for styling passes not related to selectors.

Unit test the selector compiler.

Compile 'complex' selectors (ones with multiple tests but no tree traversal).

I'm currently using a temporary datastructure which will be easily
replaced with curried functions later.

Unit test the style index.

Improve parsing API.

Now compound selectors have their own type, aiding indexing and maybe interpretation.
Now only one selector is associated with outputted style rule.

Add and pass some unit tests.