Port selector matching to use atoms.

Removed the Lowercase new type. This does mean that this
code path is slightly slower for now. This is because
find_equiv() doesn't work with atoms in hash tables, due to
the hash value being different than that of a string. However
it also means the removal of some unsafe code, and a better
long term solution will be to precache the lower case versions
as atoms.
Merge commit 'refs/pull/3029/head' of https://github.com/servo/servo

Conflicts:
	src/components/style/selector_matching.rs