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/values/generics/flex.rs

@@ -12,6 +12,7 @@
     ComputeSquaredDistance,
     Copy,
     Debug,
+    Parse,
     PartialEq,
     SpecifiedValueInfo,
     ToAnimatedValue,