Use Libsodium as an alternative ed25519 provider

[sappenin]

The library currently relies upon https://github.com/str4d/ed25519-java for its ed25519 implementation. We should consider using a Java implementation of libsodium (e.g., if this is on the classpath or a native implementation is available, the library might use that; else use str4d).

See https://download.libsodium.org/doc/bindings_for_other_languages/