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.
