Refactor ip packet

[thvdveld]

I refactored the IpPacket struct, such that we can add more extensions headers to it for IPv6

Previously it was not possible to create packets that had hop-by-hop options (for emitting). Reading hop-by-hop options was no problem though. However, I need to be able to emit hop-by-hop for RPL, which with these changes would be easier to add.

The only downside of this, is that we need alloc, or we could change it such that Ipv6HopByHop has a heapless Vec instead. This would then increase the size of the IpPacket struct.

I think this approach is better than #752, since now the structure of the IP packets is better represented than in #752.

[thvdveld]

I think I have figured out a better way of doing this. Instead of adding the hop-by-hop data to the IpPacket (in the processing stage of packets), the hop-by-hop data should just be added when in the dispatching stage of the packets.