[icxtunnel] add userspace netstack frontend

[dpeckett]

Basically wrap up a net.PacketConn in such a manner it can be treated as a physical ethernet interface for icx.

Even without vectorization it's not terrible.

=== RUN   TestICXNetwork_Speed/Speed
    icx_network_test.go:178: Downloaded 104857600 bytes in 1.84536179s → 56.82 MB/s, 454.58 Mbit/s (0.45 Gbit/s)
=== RUN   TestICXNetwork_Speed/SpeedParallel
    icx_network_test.go:235: 8 streams × 104857600 bytes each: 838860800 bytes in 9.820559846s → 85.42 MB/s, 683.35 Mbit/s (0.68 Gbit/s)
    ```