bench: Compute a better average time per round
bench: Compute a better average time per round

We're just doing a simple "total elapsed time / number of rounds", but
we should do a little bit better than that, by discarding the maximum
and minimum timing between all rounds.

criptych/graphene,ebassi/graphene,criptych/graphene,criptych/graphene,criptych/graphene,ebassi/graphene