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
auto merge of #4173 : Manishearth/servo/a-more-dom-struct, r=kmc

Now `#[dom_struct]` also generates Reflectable impls, and there's another lint to ensure that a DOM struct only contains one bare DOM field (as the first field) or a Reflector.

A lot of this was generated by sed -- each autogenerated change has its own commit for easy review; these will be squashed later.