KeyPair::new invariant check

[mversic]

When constructing a new KeyPair out of public and private key, it is not verified that the keys actually make a pair, i.e. the given public key doesn't have to correspond with the given private key. The KeyPair::new method should check that this invariant is maintained by the KeyPair container. I would suggest that this can be done by signing some dummy payload with the private key and verifying the signature with the public key. Performance of this check is not of big consideration because this constructor is rarely used and most often cached in the application