The behavior of attrs field and handling attributes

[zjp-CN]

The attrs field in this tool only includes kanitool.

But note there are more behavior when handling attributes:

• (function) item attributes are considered through source code representation
  • if a macro generates function and its attributes, this tool will catch attributes (e.g.)
• kainitool attributes are also handled in structured way, because it's more reliable to know the attribute behavior while parsing attributes strings are needless
• also note there is a disccusion on attributes representation in rustdoc-json: Improve Item::attrs to not use debug representation. rust-lang/rust#137645