Investigate bringing in optimisations in from cloudflare/bn256

[jeffallen]

This is a tracking issue to remind us to watch cloudflare/bn256#23 and bring those changes into Kyber and possibly make use of them if possible.

They are vartime, so we need to consider if we can safely use them.

[jeffallen]

See also #440, which is another vartime issue.

[AnomalRoil]

@Robingoumaz