PKCS feature flag doesn't activate pkcs8/std flag.

[FaustXVI]

When activating the pkcs8 feature, the pkcs8/std flag is not activated thus read_public_key_der_file nor read_public_key_pem_file (when pem is activated) are available.

I am new to rust and even more to feature handling so I don't feel confident enough to make a PR. My understanding is that adding "pkcs8?/std" to the std feature should do the trick.

[tarcieri]

Yes, sounds good. Please open a PR.