Add CBC-static-IV CPA secret suffix attack

[unicornsasfuel]

There is an attack on ECB where a secret appended to user input, encrypted, and disclosed in encrypted form can be obtained in plaintext form by bytewise brute force with carefully aligned boundaries. This attack is already implemented as cryptanalib.ecb_cpa_decrypt.

This attack ALSO applies to CBC mode when a static IV is in use. It's a common flaw to use a static IV with CBC mode, and as such this is a very practical attack. We should add this attack to cryptanalib.