Added Serde feature and optionnal dependency

[VirgileHenry]

I needed to be able to serialize and deserialize Subdivided<T, S>, so I forked the crate and added serde as an optional feature.

Since others may have similar needs, and this has no impacts on anyone not using this feature, I told myself that it would be nice to merge this into the main crate!

Of course, if you think this is not a good idea or have arguments against it, feel free to reject this request.

Cheers,

• Virgile

[VirgileHenry]

After further investigation, I've found that the Subdivided type use usize internally.
This can cause issues when serializing and deserializing on different architectures.

For this change to be sound, more work need to be done. Can all the usize be replaces to fixed size variants, such as u32 ?