An implementation of freertr's tapInt in Rust using the
libc and nix crates.

Improvements to the original:

 --iface_name <str>      Name of the interface
 --laddr <socket addr>   Listen address
 --raddr <socket addr>   Remote address
 --addr <ip addr>        Address to add to the interface (in CIDR format)
 --daemonize             Run as a daemon

 --addr can be used multiple times to add multiple addresses to the interface

### Features
- `crypto`: encryption using XChaCha20Poly1305. The key must be 32 bits long, and should be the same on both sides of the tunnel.