Make some minor improvements to the code base

[adamreichold]

• [x] Bump locked Cargo dependencies
• [x] Extend Clippy coverage to include all targets
• [x] Add ElementRef::attr forwarding method

Our code base is littered with calls to element.value().attr("attribute") and this little method would make this much more readable by shorting it to element.attr("attribute").

Generally, I would also consider impl Deref<Target=Element> instead of impl Deref<Target=NodeRef> but that would be incompatible and more intrusive.

[cfvescovo]

LGTM