Use new attribute syntax everywhere

[Manishearth]

#2569 changes inner attribute syntax. While solving #13476 I realized that there were a lot more occurrences of the old syntax throughout the repo. I guess these should be fixed.

[Manishearth]

The above commits were generated with sed, so I'm not entirely sure if they are valid.

[Manishearth]

Not yet PRing since this is based on #13477

[huonw]

Feel free to submit a PR with the commits from #13477 included, iirc Github will automatically drop them from the diff if that PR lands first (and if the new PR lands first then... both sets of features have landed).

[Manishearth]

Alright. It still says that the branches can't be automerged, but that shouldn't be a big issue i guess. Filed #13493

[steveklabnik]

With #13493 merged, I'm closing.