Update the dependencies and move to crypto_secretbox

[moiseev-signal]

• xsalsa20poly1305 has been recently deprecated in favor of crypto_secretbox (It is mostly a drop-in replacement.)
• Update to the latest version of rand (perhaps unnecessarily)
• Remove the remnants of the time when there was a C library underneath.

[dsprenkels]

Thank you for you contribution! <3

[dsprenkels]

Notes to self:

• [x] Checked all new dependencies for trustworthiness
• [x] Checked that all changes were necessary
• [x] Checked that the functionality was consistent across versions
• [ ] Release a new version