BTree supports Codable

[bogdad]

Hey,

thank you for the BTree, I use it in my other pet project and it works flawlessly!

This is an attempt to support of Swift Codable to BTree.

The pull request might be much smaller if Swift Tuple type and Void type supported Codable out of the box. With the current Swift state, tuple support can be fixed "internally", without many changes.

But SortedSet and SortedBag being BTree<Element, Void> is a bummer, as in extension that means, that BTree<Codable, Codable> should be Codable, and BTree<Codable, Void> should be Codable and it appears that is not allowed. Hence the need for EmptyValue.

I understand it is a big code change just to support Codable, so feel free to reject.
I would be very excited if we agree on a different workaround for this, looking forward to your comments!

Also,
this works with Swift 4.2, what would be the best way to change the Swift version in podspec?
Having multiple pods for different versions? To be honest, I am not very familiar with versioning pods for different Swift versions.

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