RSA keys

Depends on #558 

These are implemented such that they don't depend on the backend. This
means we don't have to worry about passing an RSA key created with one
backend to a different one so much at the expense of having to create a
backend specific context on demand.

This is slightly non-trivial in (at least) OpenSSL as there are 3
additional derived parameters kept in its RSA struct. They aren't
difficult to generate but it requires adding 30-40 lines of BN_* stuff
to the backend so I'm leaving that out for now. We'll need to implement
that before we can actually do any useful operations with the keys.

This also adds a loader for some of the PKCS #1 test vectors. It only
extracts the 10 key pairs from pss_vect.txt currently be should be
extenable to include the example signatures and other files later.