These will be useful for implementing various things. They don't do any
caching at the moment, but that might become valuable in the future.
To facilitate this change, I also made it possible to abort a tree walk
with for_each_in_subtree() by returning IterationDecision::Break from
the callback.
This patch adds the CharacterData subclass of Node, which is now the
parent class of Text and a new Comment class.
A Comment node is one of these in HTML: <!--hello friends-->
Since these occur somewhat frequently on the web, we need to be able
to parse them.
This patch also adds a child rejection mechanism to the DOM tree.
Nodes can now override is_child_allowed(Node) and return false if they
don't want a particular Node to become a child of theirs. This is used
to prevent Document from taking on unwanted children.
This is definitely not the ideal ownership model here, but it's
something we'll have to iterate on as the engine grows.
At least this prevents us from leaking the entire world. :^)
