cssmatcher: Speed up common matching
cssmatcher: Speed up common matching

first-child and last-child are the most common usages of the nth-child
machinery, so special-casing them makes sense.

chergert/gtk,grubersjoe/adwaita,Adamovskiy/gtk,jigpu/gtk,jadahl/gtk,davidgumberg/gtk,jadahl/gtk,jigpu/gtk,alexlarsson/gtk,alexlarsson/gtk,chergert/gtk,chergert/gtk,jigpu/gtk,grubersjoe/adwaita,chergert/gtk,Adamovskiy/gtk,Adamovskiy/gtk,Adamovskiy/gtk,Adamovskiy/gtk,davidgumberg/gtk,grubersjoe/adwaita,alexlarsson/gtk,chergert/gtk,davidgumberg/gtk,chergert/gtk,jadahl/gtk,chergert/gtk,alexlarsson/gtk,Adamovskiy/gtk,Adamovskiy/gtk,davidgumberg/gtk,grubersjoe/adwaita,alexlarsson/gtk,jigpu/gtk,davidgumberg/gtk,davidgumberg/gtk,jadahl/gtk,Adamovskiy/gtk,jigpu/gtk,jadahl/gtk,jadahl/gtk,davidgumberg/gtk,grubersjoe/adwaita,jigpu/gtk,jadahl/gtk,davidgumberg/gtk,jadahl/gtk,alexlarsson/gtk,jigpu/gtk,chergert/gtk,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,jigpu/gtk,alexlarsson/gtk,alexlarsson/gtk