This is the basic structure of the stuff. Following patches will fill
the gap between Gecko and Servo on value generating, and finally hook
it into InspectorUtils.
MozReview-Commit-ID: KNLAfFBiY6e
--HG--
extra : source : e9e5d72857746710ead3cd42481b805efc771389
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.
MozReview-Commit-ID: LI7fy45VkRw
--HG--
extra : rebase_source : a765e43b0c615e5f47bddb90ba6fa24bfc06959e
extra : source : 60812c1b190d90602bc6d49477fe1f558a53cd51