WIP: XChaCha-Poly1305 construction

[ghost]

This PR to exposes the XChaCha20-Poly1305 construction

TODO:

• [x] XChaCha-Poly1305 bindings Implementation
• [x] Generic nonce/1
• [x] Tests
• [ ] Docs
• [ ] Beautify

[ghost]

@jlouis ~Could you look at the last push? It's a broken test... but failing to understand why.~

Nevermind ;)

[ghost]

@jlouis this is ready for review... I've left the last task in my description unchecke anticipating some cleanup per a review.

[jlouis]

Closed on request of the creator