Keep pre-warmed encryption state #55

Encryption goes faster when you keep state between invocations of the cipher. For example, the OpenSSL EVP API allows this via the EVP_CIPHER_CTX_reset method, which resets the cipher state for a new operation, but keeps what is reusable.

On the one hand, this would probably make things faster. On the other hand, it's in tension with #54 because the state object would have different contents depending on the back-end library, something that could be difficult to achieve especially without dynamic allocations.