Following upstream libaegis 0.10.5 (874f884): the absorbing-mode
unauthenticated pair is deprecated in favor of the pure keystream XOR.
- stream_xor(key, nonce, data, into=None): same function encrypts and
decrypts; output equals data XOR stream(key, nonce); supports in-place
use via into (same buffer semantics as elsewhere).
- encrypt_unauthenticated/decrypt_unauthenticated kept for the old
format but now emit DeprecationWarning; removed from README (stream_xor
documented under Keystream generation instead).
