Namespace hierarchy aware compare

We should be able to detect the fact that a namespace is available on a parent element to verify equality.
For instance:

```xml
<root xmlns="toto">
  <foo>Hello</foo>
</root>
```

Should be equal to 
```xml
<root>
  <foo xmlns="toto">Hello</foo>
</root>
```

And also equal to
```xml
<root xmlns:t="toto">
  <t:foo>Hello</t:foo>
</root>
```