cryptoScalarMult takes inputs in the wrong order

[timmc]

DiffieHellman.Lazy.cryptoScalarMult accepts parameters named publicKey, secretKey, but the underlying implementation expects the secret key first. If you use the documented argument order, key agreement doesn't work.

https://github.com/terl/lazysodium-java/blob/64dc78554f45eb9b81e74354d61eba4e3da2452b/src/main/java/com/goterl/lazysodium/interfaces/DiffieHellman.java#L49

[timmc]

Relatedly, it also might be wise to create subclasses of Key with names like SodiumBoxPublicKey. Even if the method is documented correctly, having everything be called Key makes it easy for a caller to mess up. (I'd be happy to submit a PR for this, as I've done something similar in my own use of lazysodium.)