3.8.0

New in release 3.8.0:

1. There is a new ECPrivateKey method 'sharedSecret(pubKey:cofactor:)'
   which implements the basic Diffie-Hellman key agreement primitive.
   It multiplies another party's public key (a curve point) by our own private key (an integer),
   and return the resulting point's x-coordinate as result.

   The method was there all the time as an internal helper method, but now it is public.

2. The deprecated ECPrivateKey method 'keyAgreement' is removed,
   but it still exists under the name 'x963KeyAgreement'