WIP: sessions: Zero out the plaintext buffers when decrypting.
This MR adds support to zero out some buffers holding sensitive data. While not many cases have been found, we could expand this further for example if the Pickling mode would take ownership of the pickle passphrase.
If the pickling mode would be modified in that way, the library user wouldn't need to worry about zeroizing the passphrase himself.
This closes #35 (closed).
Edited by poljar