IPv6 support (work in progress)

Started working on adding IPv6 support (see #43). Right now there is very little code, but I wanted to put it here so that there is a central place for discussion for those interested.

Right now the code only handles incoming `tcp` and `udp` payloads when there are no `ipv6` extensions present.  I am looking into implementing `icmp6` echo handling next.
You are right, this does not compile in 4.01.0.  Submitted a fix in #91.