Adds smart selector restructuring.

Adds smart selector restructuring.

The algorithm traverses tokens list backwards, picking up any properties
on its way. If a property already exists and has the same value, it adds
the current token to the list of matching that property, otherwise one
of two things happens:

* The property is dropped from the list if there's been only one token
  associated with it.
* It tries to move properties around if it results in a shorter content.