Also adds HeapSizeOf implementations/derive for some types. I've used "Cannot calculate Heap size" as a reason everywhere, because my imagination is rather limited. If you'd like me to change this message for specific types, please write something like this: "Trusted - Cannot calculate Heap size for Trusted" so that it would be easier for me to replace them through a script :)
Source-Repo: https://github.com/servo/servo
Source-Revision: a03616f379c255cc6c9b6e1d04dd7d98bd9926ce
