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.
No, I do not think so.  If I understand correctly the 4.02.0 module alias stuff only refines the *typing* of expressions such as `module M = ...`.