Review. Removed a handful of low-priority TODOs, added others due to code analysis.
It looks like a lot of changes, but most of it is just identifier renames.

The more relevant stuff is
- Moved packet fragmentation from 'Translate the Packet' to 'Send Packet', because it feels more at home there. Also, Translate is complicated enough as it is.
- Removed several redundant structures: ipv6_pair, ipv4_pair, tuple_addr.
- Translate routes earlier again. This helps the stats functions succeed.

I haven't updated the userspace app.