Make Node::is_in_doc O(1)

Added a flags variable inside Node to represent boolean flags, with
is_in_doc being the first of them. It is updated whenever a node is
appended or removed from a parent.

This patch is for:
https://github.com/mozilla/servo/issues/1030
auto merge of #1427 : brunoabinader/servo/is-in-doc, r=jdm

Added a flags variable inside Node to represent boolean flags, with
is_in_doc being the first of them. It is updated whenever a node is
appended or removed from a parent.

This patch is for:
https://github.com/mozilla/servo/issues/1030