feat: move serde serialization behind the serialize feature flag

[lavafroth]

Fixes #19

Changes

• A new feature flag called serialize is created inside Cargo.toml
• Structures and enums implement serialization and deserialization only when the feature is enabled
• This feature flag is enabled by default since the main CLI requires reading serialized data
• Upstream libraries can add default-features = false in their Cargo.toml while adding poasta as a dependency which will remove serialization and deserialization capabilities and gate the save_graph and load_graph functions