Use cast instead of transmute from u64 to uint.

Fixes build error on 32-bit targets.
Upgrade Rust.
Add Hash to derived trait implementations for Atom.
Generate documentation with rustdoc
Change to derive Hash, remove Str so Equiv is not implemented.
Add the option of building with Cargo
Import macro-based static atoms from html5ever
Re-export atom::Atom at top level
Fixes for Servo's older Rust