Background information: The DOM standard defines a MutationObserver API that allows web content to receive callbacks when the page contents are mutated. The goal of this work is to implement the fundamental pieces required to support this API.
Tracking issue: servo/servo#6633 (please ask questions here)
Initial steps:
create the MutationObserver and MutationRecord interfaces with stub method implementations (:warning: TODO: implement void disconnect();, sequence<MutationRecord> takeRecords();)