Add block ciphers

Open questions:
- Presumably individual block operations should be behind a feature flag since they're low level, but we still want to be able to expose high-level cipher modes (e.g., AES-GCM) via composition. How do we do that?
- What block cipher modes do we have to support?
- Do we want to build in padding?
- Do we want to expose a non-padding variant as well?
- How do we ensure that padding is only with safe block cipher modes (e.g., don't allow PKCS11 padding w/ CBC, which is vulnerable to a padding oracle attack)