yaca_key_extract_public() public function

yaca_key_extract_public() public function

The idea is to remove yaca_key_gen_pair() and always generate using
yaca_key_gen(). The latter will always generate private keys for
asymmetric types. Public key will be extracted (if needed) using the
function implemented here.

This approach has an advantage that we can extract public keys for
private keys imported from external sources. Previously this was
impossible.

Change-Id: I081c81eb37ed3267518aac21a9bf36063ef0e901