Add popCount to Data.Bits

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

The proposal was discussed and agreed upon in this thread: http://thread.gmane.org/gmane.comp.lang.haskell.libraries/16073

Trac metadata

Trac field	Value
Version	7.2.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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