use ipv6 addressing scheme where both sides agree on addresses

[majestrate]

with ipv6 we may have enough room to put the first N bytes of a loki address as the last N bytes of an ipv6 address. this would make it so both sides agree on what each other's ip addresses are. this could help address incompat issues with webrtc partially.

[jagerman]

I've registered the ipv6 ULA prefix fd2e:6c6f:6b69::/48 (i.e. fd followed by ".loki" in hex) for Lokinet for this purpose on https://ula.ungleich.ch/, which seems to be the current best effort at ULA registrations.

The idea for addressing is that we would allocate this /48 to the tun device (leaving us 80 bits of addressing space), take the first 4 bits as an address type (where 0 = .loki, 1 = .snode, and anything else is reserved for future use), and the remainder is the first 76 bits of the pubkey. So for a remote pubkey of (hex) abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789 we'd allocate an address of:

fd2e:6c6f:6b69:abc:def0:1234:5678:9abc  (if a client)
fd2e:6c6f:6b69:1abc:def0:1234:5678:9abc  (if a snode)

For the tun itself, then, we would set the main tun IP and range of fd2e:6c6f:6b69:abc:def0:1234:5678:9abc/48 (or the same with the extra "1" if a snode).

[jagerman]

Also note that, in order to make this work (especially on macos) we need to adjust where we handle loading/generating the identity keys so that we can get it after a call to Context::Configure but before Setup() (because we need to set up the tun device via Apple API bits before Setup() so that Setup() has a configured device already).