AttrValue's s/from_tokenlist/from_serialized_tokenlist/
Avoid duplicated tokens in AttrValue::from_serialized_tokenlist
Added AttrValue::from_atomic_tokens & Element::set_atomic_tokenlist_attribute
Added Element::get_tokenlist_attribute
DOMTokenList::check_token_exceptions now returns an Atom
Implement DOMTokenList.add
Implement DOMTokenList.remove
Implement DOMTokenList.toggle
Updated web-platform-tests results
auto merge of #4353 : brunoabinader/servo/domtokenlist, r=Ms2ger

Specs:
https://dom.spec.whatwg.org/#dom-domtokenlist-add
https://dom.spec.whatwg.org/#dom-domtokenlist-remove
https://dom.spec.whatwg.org/#dom-domtokenlist-toggle
https://dom.spec.whatwg.org/#concept-dtl-update
https://dom.spec.whatwg.org/#concept-ordered-set-serializer

Closes #3138.