layout: Remove `FontStyle` in favor of using the font style struct
directly, and optimize `get_layout_font_group()` to use a small vector.

Seems to be a 38% layout win on a site I tested with a lot of text.
auto merge of #3697 : pcwalton/servo/get-layout-font-group, r=glennw

Seems to be a 38% layout win on a site I tested with a lot of text.

Other browser engines typically do not duplicate the information in the font style struct. `FontStyle` actually predates @SimonSapin's CSS selector matching library. It's time to get rid of it!

r? @glennw