Is there an already-written `BitCount` method for big.Int? There doesn't seem to be one in math/big.

Obviously I will write one myself if not - does anyone have one already written?

I want the number of set bits in the number. Like [Java BigInteger.bitCount()](http://docs.oracle.com/javase/1.4.2/docs/api/java/math/BigInteger.html#bitCount%28%29).