Switch to ring-xous

[Niederb]

The ring crate is the main implementation for cryptographic primitives in the rust ecosystem. Our problem for the WebAssembly use case is that for elliptic curve cryptography it still relies on implementations in assembly. Also see this issue: briansmith/ring#918

ring-xous is a fork of ring that creates a pure rust implementation of ring.
The blog-post gives quite a good introduction to the problem and how they solved it:
https://www.bunniestudios.com/blog/?p=6521

[Niederb]

Done