Properly support template elements in DOM

[nielsdos]

The template element in HTML 5 is special in the sense that it does not
add its contents into the DOM tree, but instead keeps them in a separate
shadow DOM document fragment. Interacting with the DOM tree cannot touch
the elements in the document fragment.

[Girgias]

Seems reasonable, I've got some questions

[Girgias]

Also feel free to already merge the first two commits if you want

[Girgias]

Thanks, looks great!

[nielsdos]

This still needs a bit more work apparently; and I need to check if the new code works causes problems with nsDef being filled in...

[nielsdos]

I realized that the destruction solution was not good yet because of the following two reasons:

• Destroying a template can be done indirectly
• Destroying a template can be done through another extension (e.g. simplexml)

So it doesn't suffice to add the logic for destroying templates in the dtor of dom. Instead, we have to callback from ext/libxml to destroy our associated template content. To accomplish that, I introduce the ability for namespaces to signal additional information to ext/libxml. We already used the _private field from libxml to cache information for dom, but we can use a few lower bits using pointer tagging to set some flags as well. We use the flag bits to signal that a custom cleanup routine needs to be called in case the node is a special type of node, which makes sense to signify that via a namespace. This mechanism can be expanded upon later too.

[nielsdos]

Upon merging I will keep the first two commits separately and then squash the others into a third one.

[Girgias]

The hook idea makes sense, I had also forgotten about SimpleXML

[Girgias]

Should this return a uint32_t instead?

[nielsdos]

Hmm probably