Define crate features

[pronebird]

• Add ipnet and ipnetwork features
• Add ipnet and ipnetwork to default features
• Add serde feature

ps: not sure what should be in default features, ipnet or ipnetwork or nothing at all? 🤔

[sticnarf]

I'd like to keep only ipnet in the default features. While default features should ideally be minimal, I prefer keeping ipnet enabled by default for historical consistency.

Regarding the feature definitions, optional dependencies are already implicit features, so there’s no need for separate definitions.

[pronebird]

Ok that's fine. Closing this PR then.