Bind crypto_aead_chacha20poly1305_ietf_encrypt()

[cfcs]

Hi, I need the Authenticated Encryption with Additional Data construction bound:

• crypto_aead_chacha20poly1305_ietf_encrypt()
• crypto_aead_chacha20poly1305_ietf_decrypt()
  (note that the _ietf_ functions are similar to crypto_aead_chacha20poly1305_en/decrypt functions, but take a longer nonce (96 bit as opposed to 64bit))

I can work on a PR with my own attempt, but you seem like you have a much better grasp of the Ctypes toolchain than I do, so it would be great if you have time.