Optional serde support

[chrjabs]

Description of the Contribution

Implement serde::Serialize and serde::Deserialize behind a serde feature.

resolves #270
closes #271

PR Checklist

• I read and agree to CONTRIBUTING.md
• I have formatted my code with rustfmt / cargo fmt --all
• Commits are named following conventional commits
• I have added documentation for new features
• The test suite still passes on this PR
• I have added tests for new features / tests that would have caught the bug this PR fixes (please explain if not)
• If this PR contains breaking changes, it is against the next-major branch, not against main