Because ring does not currently support RSA keys generation, the default
algo must be adapted depending on whether or not the standalone feature
is activated.
Since it is planned to add a "standalone" feature that will replace
OpenSSL by crates not linking to any external library, it is required to
abstract all the OpenSSL specific types. This is a huge work and
therefore is divided in several steps. This first one is dedicated to
public and private keys.
rel #2