This is a version of ChaCha20-Poly1305 that takes a 24-byte nonce,
making the nonce suitable for random generation. It's compatible with
the AEAD of the same name in libsodium.

Change-Id: Ie8b20ba551e5a290b390d362e487f06377166f4c
Reviewed-on: https://boringssl-review.googlesource.com/30384
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: David Benjamin <davidben@google.com>