Ipv6: Refactor

- Split NDP handling to module Ndpv6.
- Merge Ipv6_core and Ipv6.
- Implement a simpler IP signature.  We go back to the original
  sig (involving `allocate frame` and `write`), but we do address
  resolution in `write` instead of `allocate_frame`.
Ipv6: Send RS on initialisation