extmod/modlwip: Support IPv6.

With these changes IPv6 works on the rp2 port (and possibly others that use
the lwIP socket implementation).

Things that have been tested and work:
- Neighbour solicitation for v6 link local address.
- Ping of v6 link-local address.
- Receiving a SLAAC address via router advertisement.
- Ping a v6 address allocated via SLAAC.
- Perform an outgoing connection to a routed v6-address (via default
  gateway).
- Create a listening IPv6 wildcard socked bound to ::, and trying to access
  it via link-local, SLAAC, and IPv4 (to ensure the dual-stack binding
  works).

Things that could be improved:
- socket.socket().getaddrinfo only returns the v4 address.  It could also
  return v6 addresses (getaddrinfo is actively programmed to only return a
  single address, and this is the v4-address by default, with fallback to
  the v6 address if both are enabled).

Signed-off-by: Felix Dörre <felix@dogcraft.de>

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