IPv6 APIs rework

[Robin Jarry]

Cutting down the quoting a bit.

I understand the arguments against having an unaligned packed struct. 
But it is precisely what we have in the current code base. All IPv6 
addresses are uint8_t[16] arrays. And nobody ever complained about it.

Since this rework is already massive, could we proceed in steps?

First, I can replace all uint8_t[16] arrays by an unaligned packed 
struct which is 99% of the work.

Second, *if there is a real and measurable performance gain*, change 
that structure to remove explicit alignment.

Does this sound like a good plan to everyone?