strip leding zero bytes from PrivateKey secret

[NicolaLS]

Instantiating two private keys using e.g. \x00\xff and \xff as the secret should not create different (property wise, not instance wise) private keys in the context of a high-level interface.

I understand that the underlying functions behave differently but on a high level I'd expect the public key derivation sG to be the same since the integer value of s is the same no matter how many leading zeros the secret bytes have.

Private keys are often serialized with a 0x00 leading zero byte padding, it would be nice to just pass this to PrivateKey() instead of having to strip that.