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.

components/style/properties/mod.rs.mako

@@ -774,6 +774,13 @@ pub mod longhands {
 //                Fantasy,
 //                Monospace,
             }
+            impl FontFamily {
+                pub fn name(&self) -> &str {
+                    match *self {
+                        FamilyName(ref name) => name.as_slice(),
+                    }
+                }
+            }
             pub type T = Vec<FontFamily>;
         }
         pub type SpecifiedValue = computed_value::T;