The alias is left there temporarilly and will be removed completely in a later commit where
also components/style/gecko/generated/structs_{debug|release}.rs are re-generated (they still
use the old alias).
This file has become quite bloated lately. This commit deletes that file in
favor of a set of submodules.
The only noticeable change apart from code move, is converting deep_clone_foo
methods into a trait.
It also unifies logic related to different style rules in the same place.
There's some missing work, specially related to font-face and counter-style, but
I think this is worth landing in the meantime.
