Test Ed25519 compatibility

[clehner]

A verification error was observed in verification compatibility between the ring Ed25519 and ed25519-dalek implementations. This issue is to track how this is happening and adding tests to ensure compatibility between the Ed25519 implementations.

[clehner]

The reported error was apparently due to something other than the signature verification. So we assume there is no incompatibility. But the tests should probably be improved anyway to catch any potential incompatibility.
This issue can remain to track testing signing with one of the ed25519 implementation and verifying with the other.
Currently ssi can only be compiled with one implementation or the other. Existing test vectors were generated using the ring implementation, I think. Possibly an example script or Rust build script could be run in the CI with each implementation to generate test vectors, and then again to verify each test vector with each implementation.