style: Improve #[derive(Parse)].

I want to do this so that I can get rid of Either<>. The reasons for getting rid
of either are multiple:

 * It doesn't generate as nice C++ code using cbindgen.
 * It isn't that nice to use either from Rust.
 * cbindgen has bugs with zero-sized types.

I started using this for ColorOrAuto and a few others, for now.

Differential Revision: https://phabricator.services.mozilla.com/D19844

components/style/properties/longhands/inherited_ui.mako.rs

@@ -60,7 +60,7 @@ ${helpers.single_keyword(
 ${helpers.predefined_type(
     "caret-color",
     "ColorOrAuto",
-    "Either::Second(Auto)",
+    "generics::color::ColorOrAuto::Auto",
     spec="https://drafts.csswg.org/css-ui/#caret-color",
     animation_value_type="AnimatedCaretColor",
     ignored_when_colors_disabled=True,