Switch to the new Cargo.lock format

The way the Cargo.lock file is encoded on stable Rust is quite unfriendly
to Git when multiple pull requests change that file at the same time,
how version numbers are everywhere and all checksums are in the same place
often produces merge conflicts for nothing.

This patch brings the new Cargo.lock format to Servo.

	rust-lang/rust#63579

The main caveat is that for now, cargo-tree and similar tools won't work
anymore.

I checked that the duplicate crate tidy check still does its job though.

Author: nox