This introduces new aes_encrypt_iter() and aes_decrypt_iter()

functions in djblets.secrets.crypto, which can operate on iterables.
aes_encrypt_iter() iterates through an iterable that yields byte or

Unicode strings to encrypt. It then yields the encrypted blocks. It's

smart enough to include the IV in the first block and the finalizer in

the final block.
aes_decrypt_iter() iterates through an iterable of encrypted byte

strings. It yields the decrypted blocks.
These will be used internally for encrypted caching, but can also be

useful for things like streaming data to files or HTTP responses.

Unit tests pass.