Expose `Ed25519Error` type externally or add mapping of it to `RecryptErr` type.

We recently changed the library to expose the `RecryptErr` type which was accidentally not being exposed out of the library. Now I noticed that we have the same problem with the `Ed25519Error` type. From what I can tell it's only ever used as part of the `SigningKeypair` methods, so maybe the better path forward here would be to get rid of it in favor of the `RecryptErr` type.