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.
CGDataProviderCreateWithData just wraps the underlying buffer. The
underlying buffer needs to be kept around until the data provider is
freed. Adding the buffer to the FontTemplateData struct ensures it
sticks around.
