This commit introduces the `serde` dependency, which we will use to serialize messages going between processes in multiprocess Servo. This also adds a new debugging flag, `-Z print-display-list-json`, allowing the output of display list serialization to be visualized. This will be useful for our experiments with alternate rasterizers. r? @metajack Source-Repo: https://github.com/servo/servo Source-Revision: ef9715203edf0a280d019b6e8823666f0e7020be