Update dependencies and address clippy lints

[lukehsiao]

style: fix all clippy lints

Suggestions were found by running

cargo clippy --all-targets --all-features -- -D warnings

using

clippy 0.1.69 (84c898d 2023-04-16)

---

build(deps): update atom_syndication to v0.12

[lukehsiao]

It would be great to get a new release of syndication after these dependencies are updated.

Otherwise, the current version on crates.io (v0.5.0) is still using atom_syndication v0.6

https://github.com/rust-syndication/syndication/blob/0.5.0/Cargo.toml#L12