Resolve build issues

# Project Improvement

Currently the following crates do not compile with the latest stable rust compiler:
- [x] oxide-auth-db
- [x] oxide-auth-iron (the example fails to build)
- [ ] oxide-auth-rocket

## Other context

Making oxide-auth-rocket compile should probably happen as part of upgrading to version 0.5 of that crate: #131

# Tracking pull request

- oxide-auth-db - #220
- oxide-auth-iron - #221

該 issue 列出了三個建構失敗的 crate：oxide-auth-db 和 oxide-auth-iron 已有拉取請求（#220、#221），剩下 oxide-auth-rocket。首先檢查儲存庫的 CI 日誌或在每個 crate 上執行 `cargo build` 以查看確切的編譯器錯誤。對於 oxide-auth-rocket，升級到 0.5 版本在 issue #131 中追蹤；請檢查該 issue 以及該 crate 的相依項。「Done」意味著所有三個 crate 都能使用最新的穩定版 Rust 編譯器編譯。

由索引模型根據 Issue 內容生成。