Pure dart implementation of ed25519

[Tougee]

Because there is no Dart library that supports translation between x25519 and ed25519, so I backported the Go implementation of edwards25519.

[xni06]

I can confirm that I am able to:

• consume this library
• create and verify a signature on both an iOS and Android devices
• run a Dart unit test

Thanks!

  test('verify signature', () {
    final ECKeyPair keyPair = Curve.generateKeyPair();
    final message = Uint8List.fromList(List.generate(32, (index) => index));
    final signature = Curve.calculateSignature(keyPair.privateKey, message);
    final verified = Curve.verifySignature(
      keyPair.publicKey,
      message,
      signature,
    );
    expect(verified, true);
  });