Proposal: Add Data.Bits.popCount

Hi! Now when we have a fast popCnt# primop* it would be nice to expose it to the world through an official API. I propose we add this method to the Bits class in Data.Bits: -- | Return the number of set bits in the argument, know as the -- population count or the Hamming weight. popCount :: a -> Int I will provide a default implementation, which means that this change won't break any existing user defined instances. All the instances for the basic types (Ints, Words, etc) will use the primops. * These primops compile to a single POPCNT instruction if the user compile using -msse4.2 and a fast lookup table based implementation otherwise. Discussion period: 2 weeks Cheers, Johan