[Development] Backporting the Keccak change

[Oswald Buddenhagen]

On Wed, Aug 30, 2017 at 12:45:44PM -0700, Thiago Macieira wrote:
>  a) revert the 5.6 backport of 88a8feeacb9bdaff9ee06164424e407eb904cd10 so 
> that 5.6.x will forever calculate Keccak, not SHA3;
> 
>  b) additionally backport 12c5264d9add1826d543c36d893db77262195fc6 to both 5.6 
> and 5.9, with the proper binary compatibility notices, so that people who need 
> to can adapt their code to calculate Keccak. It won't be pretty, but it will 
> work.
> 
> I'm actually leaning towards (a) for 5.6 and (b) for 5.9.
> 
b) isn't legitimate for 5.9, either, as we can't add new api in a patch
release (forward compat promise). so it would be a) for 5.9, too. the
unfortunate effect is that this is a behavior change against 5.9.0, but
5.9.2+ will certainly have a longer lifetime than 5.9.x so far.

if we wanted to be really conservative, we'd leave the old meaning of
the sha3 constants and introduce realSha3 (or something to that effect)
instead, in 5.10+. keccak aliases would be also provided for a smooth
migration.