Use BoringTun for the WireGuard implementation

[6c67b9]

https://github.com/cloudflare/boringtun

Why?

BoringTun is an implementation of the WireGuard® protocol designed for portability and speed.

Mature:

BoringTun is successfully deployed on millions of iOS and Android consumer devices as well as thousands of Cloudflare Linux servers.

Less bugs:

https://mullvad.net/en/blog/announcing-gotatun-the-future-of-wireguard-at-mullvad-vpn

Faster:

cloudflare/boringtun#382