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
Most of types just derive it using proc_macro directly. Some of value
types need manual impl.
In my current plan, this new trait will be used in bug 1434130 to expose
values as well.
Bug: 1455576
Reviewed-by: emilio
MozReview-Commit-ID: LI7fy45VkRw
