proposal: spec: add support for int128 and uint128

[mei-rune]

[ianlancetaylor]

Can you provide a real-life use case?

[twoleds]

it's good for UUID, IPv6, hashing (MD5) etc, we can store IPv6 into uint128 instead byte slice and do some mathematics with subnetworks, checking range of IP addresses

[minux]

These use cases are not strong enough to justify adding 128-bit types,
which is a big task to emulate it on all targets.

1. MD5 is not secure anymore, so there is little benefit adding types to
   store its result.
2. How often do you need to manipulate a UUID as a number rather than a
   byte slice (or a string)?
3. The other use cases can be done with math/big just as easy.

Also note that GCC doesn't support __int128 on 32-bit targets and Go do
want consistent language features across all supported architectures.

[twoleds]

I agree with you there aren't a lot of benefits for int128/uint128, maybe a little better performance for comparing and hashing in maps when we use uint128 for storing UUID/IPv6 because for byte slices or string we need do some loops and extra memory but it isn't important I think

[mei-rune]

I stat all interface flux of a device in one day.