Remove non-IETF [x]chacha20poly1305 constructions

[zx2c4]

Merging #461 was a step in the right direction, since it clearly defines the xchapoly functions in terms of the other constructions.

But really we should remove non-IETF [x]chacha20poly1305 constructions.

The padding they add could be an important security feature. More importantly, every place else implements the IETF version, so keeping two around hampers interop. A the very least, you could prevent new code from linking against those functions while still maintaining binary compatibility with old code. Alternatively, promote the IETF versions to not have the _ietf part of the name, and demote the non-IETF versions to have _nonietf part of the name.

Or some other good solution. But let's get thinking about interop.

[tarcieri]

At the very least, I think it would be nice if everyone defining an "XChaCha20Poly1305" based the construction on the IETF version, so we don't have to worry about two variants of that.

[jedisct1]

The non-IETF versions are not going to be removed or renamed. It would be a breaking change, breaking existing applications, so this is not going to happen before a major revision.

But yes, other libraries implementing XChaCha20Poly1305 (now based on the IETF version) would be neat.