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.
It should replace `match foo.to_ascii_lower().as_slice() { ...}`
@Manishearth I changed map.get to map.find in the lint to work around an ICE:
task 'rustc' panicked at 'couldn't find node id 0 in the AST map'
Does this look OK?
