Implement Serialize and Deserialize for Tree

[Tirzono]

Would it be possible to implement serde's Serialize and Deserialize for the Tree struct? This would allow me to generate the tree, save it to a file and load it when I need it to speed up things.

[kornelski]

Serde would be a bit problematic, because:

• nodes contain item values. You probably have the values stored elsewhere already.
• it'd expose internal implementation, and I don't want to promise that tree saved in one version can be read by another.

Maybe there could be a method to import/export Vec<(near, far, idx, radius)>?