Added better mutability, removed some clones

[arduano]

I started using this library in order to modify attributes in my HTML archiver (e.g. modify image src attributes, etc). However I noticed that this library doesn't support mutably querying elements by selectors, so instead of manually implementing it in my code I decided to fork this library instead. Along the way, I modified some other things as well.

Here is the full list of whats changed, off the top of my head:

• Node::Element now has (Element) instead of { element's fields }. This means we don't need to borrow (or clone) things across when coverting a Node into an Element.
• The above means that the number of clones went down significantly, previously it would make heaps of unnecessary clones when querying (cloning an entire tree just to get the next child element, etc).
• Made query functions return references instead of owned+cloned values. This is both better for performance, but also it lets the queries return &mut references, which is useful for editing the html
• Added a function that mutably executes a closure on all elements that match a selector, this is very useful for editing element attributes.
• Removed .into_element, instead it's .as_element which returns an option of the borrowed element inside.
• Implemented .into_node for Element for easier conversion, as well as an Into<Node> just in case.

Also, for the doctest in the remove_by function, I added an extra element in that line so that the IDE's auto formatter (not even rustfmt) doesn't strip the extra spaces from that line. VS Code automatically trims the end of each line, which makes that test fail after saving the file.

[lomirus]

Thanks for your great work. It seems good.

However, I maybe busy in the recent a few days or a week, so it may take some time for me to be able to review this PR. Thanks for your patience :)

[lomirus]

Some clippy errors need to be fixed.

[arduano]

Just need your approval to re-run the workflow