The primary reason for migrating is that these crates provide APIs for
setting the personalisation string. This enables us to depend solely on
published crates, and thus publish our own crates.
The SIMD implementations are ported from libsodium.
Closes#67.
This crate exposes both the ChaCha20Poly1305 IETF construction, and the
underlying ChaCha20 IETF primitive, removing the need for depending on
our own fork of the previous chacha20-poly1305-aead crate.