Lots of fixes and improvements from various people, thank you!

Changes

• Edition 2021
• Upgrade to Rust 1.59.0
• Treat <content:encoded> and <description> as HTML (thanks @kevincox)
• Fix bug handling MediaRSS content and encoded where MediaRSS handling incorrectly consumed the next element tree by advancing past the current scope.
• Unescape attribute content
• Support atom links in RSS (thanks @kevincox)
• Code quality improvements (thanks @rillian)
• Remove serde as a feature as we don't serialise or deserialise the model in this crate
• Improve test coverage for Ghost powered feeds