More correctly, *also* per-token. Because as it turns out, while the top-level edition affects parsing (I think), the per-token edition affects escaping of identifiers/keywords.
Previously all lints were assumed to be `#[warn]`, and we had a hand-coded list of `#[allow]` exceptions. Now the severity is autogenerated from rustdoc output.
Also support lints that change status between editions, and the `warnings` lint group.
