Import macro-based static atoms from html5ever

Instead of enum values like `atom::StaticAtom::Href`, we use a macro invocation `atom!(href)` which can be either an expression or a pattern.

Servo can build with this version of the library, without any changes to Servo itself.

**Note**: I'd like to merge the first three commits to `master`, with the last commit only on a `servo` branch that we'll drop after the next Rust upgrade.

r? @glennw