Add is_initialized() to Pkcs11

[ximon18]

I recently created a PR for the Krill project to switch from the pkcs11 crate to the cryptoki crate and in doing so I discovered a missing piece of functionality: the cryptoki crate has no equivalent of the pkcs11 crate `is_initialized() function.

Of course I can track whether or not I have called initialized myself but that's extra work. Or alternatively add an fn initialize_if_needed() or enable fn initalize() to tolerate being already initialized, which would prevent the caller from having to check and then call initialize to avoid getting an already initialized error.

[ionut-arm]

I think we should both add is_initialized and make initialize idempotent.