Port _CShims UUID to Swift

[tonyarnold]

This is a spike for my own learning to see what's involved in removing the C-backed UUID implementation in favour of a Swift version.

I'm happy to revise this, as I realise that the implementation of some of this isn't ideal.

[parkera]

One interesting security feature of our UUID implementation is that it is designed to do its work in constant time regardless of the content of the UUID. e.g., for equality checking. This is done in order to defeat timing attacks. We would need to verify we keep that behavior with a new implementation.