Support Snafu errors with generics

If possible, include the generics (https://docs.rs/snafu/0.5.0/snafu/guide/generics/index.html) in the expanded `impl Debug` block.

Start by locating the expanded `impl Debug` block and read the linked Snafu generics guide to understand the requested support. Done means the expanded implementation supports Snafu errors with generics; the payload does not name a test or specific file to run.

索引モデルが issue の本文から書いたものです。