These are translated from [monocypher](https://monocypher.org/) which
has fairly competitive performance while remaining quite simple.

Initial performance comparision:

Zig:
 Poly1305: 1423 MiB/s
 X25519:   8671 exchanges per second

Monocypher:
 Poly1305: 1567 MiB/s
 X25519:   10539 exchanges per second

There is room for improvement and no real effort has been made at all in
optimization beyond a direct translation.