Improve API for creating new elements

[matt2xu]

Instead of Element::new(name, attributes), we could do separate the text and attributes part:

1. Element::new(text) // works for any kind of element: text, comment, start, end...
2. element.add_attributes(iter) // only useful for start elements

This way, no need to pass an empty iterator for elements where attributes don't make sense (text or comment).

I would also change the Element::new to use an AsRef<[u8]> for the name, to allow faster/easier creation of elements from existing ones.

[tafia]

Looks like a good idea.

For the attributes, I'd prefer extend_attributes(iter) to consume an iterator (a-la Vec).
A push_attribute could be useful too (could be internally used by the extend version) because one does not always have an iterator at hand.

[matt2xu]

👍 I'll look into it.

[tafia]

closed by #5