Elligator transformation support

[Scratch-net]

Hi there and thanks for a good translation of nacl code to c#. I've implemented Elligator transformation in c# based on your code and added some tests, though self-testing can be easily done at runtime.
My implementation is here it is almost a direct copy of C and Go implementations.

If you'd like to include this in your library, I'd be happy to create a pull request.