Warning police
Add a hexdump function for debugging
Embed layout data directly in Node

This eliminates

    layout_data: Option<@mut ()>

and the unsafe casting around it, which was causing crashes on exit.

Fixes #762.
Remove a layer of Option on LayoutData

The individual fields are also Options, and LayoutData::new() is a fine default
value.
auto merge of #862 : kmcallister/servo/embed-layout-data, r=jdm