Change Random.Pcg to Random.Pcg.Extended

[Zinggi]

This pull request addresses #10.

I might have gone a bit overboard by changing function names, documentation and no longer exposing Uuid.Barebones (the two functions there are now exposed from Uuid), but I think those changes improve the package overall.

If you don't want to merge this because of the invasive changes, I can also publish my own fork, whatever you prefer.