fix: PrivateKey Curve bug

[aitimate]

The current *ecdsa.PrivateKey does not conform to the standard secp256k1 curve, and therefore cannot be directly used with go-ethereum or related libraries for signing transactions. If you need to reproduce the issue, you can test it using the code below:

// see: https://github.com/ethereum/go-ethereum/blob/master/crypto/signature_nocgo.go#L82
func TestCurve(t *testing.T) {
	entropy, _ := bip39.NewEntropy(256)
	mnemonic, _ := bip39.NewMnemonic(entropy)
	hd, _ := NewFromMnemonic(mnemonic)
	path := MustParseDerivationPath(fmt.Sprintf("m/44'/60'/0'/0/%d", 0))
	account, _ := hd.Derive(path, false)
	privateKey, _ := hd.PrivateKey(account)
	if privateKey.Curve != crypto.S256() {
		panic(fmt.Errorf("private key curve is not secp256k1 : %v(%v)", getCurveName(privateKey), privateKey.Curve))
	}
}