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.
Use inline(always) for trace()
Add internal plugin for creating Reflectable implementations
Remove manual impls of Reflectors (autogen)

Obtained via:

 `find . -maxdepth 1 -type f -print0 | xargs -0 sed -z -i "s/\\nimpl Reflectable for[^{]*{[^}]*}[^}]*}\\n//"`
 `find . -maxdepth 1 -type f -print0 |xargs -0  grep -lZ dom_struct | xargs -0 grep -LZ  "reflector()\\|Reflector::new" |xargs -0 sed -z -i "s/use dom::bindings::utils::{Reflectable, Reflector, reflect_dom_object};/use dom::bindings::utils::reflect_dom_object;/"`

followed by semi-automated removal of leftover imports
Remove extra spaces

Command: `find . -maxdepth 1 -type f -print0 | xargs -0 sed -z -i "s/}\\n\\n\\n/}\\n\\n/"`
Add inheritance-checking lint
Ensure that Reflectors are the first field
Fix warnings post-upgrade
gfx: to_string() -> into_string()
style: to_string() -> into_string()
script: to_string() -> into_string()