Invalid use should raise an error

[burgerdev]

I just managed to confuse the input type, it would have been helpful if this were not accepted:

let _ =
  let pub = X25519.(base |> string_of_public_key) in
  let priv = "0123456789abcdef0123456789abcdef" in (* <-- note that this should be 64 hex characters, not 32 octets! *)
  let pub = x25519 ~priv ~pub in
  print_endline pub
  (* happily prints 633d0241b2d1519ecfc84b75e1520e01fed24538f36b2d26f939ddd1f1efd73d *)