WIP IPv6 refactoring

Changes so far:

* reinclude net_if into IPv6 (`ipv6_if`)
* introduce IPv6 address module (`ipv6_addr`)<br>
  most modules that want to use something from IPv6 (the above mentioned `ipv6_if` e.g. just want to do something with IPv6 addresses so this was overdue IMHO)

Still WIP

Depends on #1680 and #1968.