code: func MutableFor(valueCount, bitsPerValue int, acceptableOverheadRatio float32) Mutable {
  	formatAndBits := FastestFormatAndBits(valueCount, bitsPerValue, acceptableOverheadRatio)
  	return MutableForFormat(valueCount, formatAndBits.BitsPerValue, formatAndBits.Format)
  }
docs: /*
  Create a packed integer slice with the given amount of values
  initialized to 0. The valueCount and the bitsPerValue cannot be
  changed after creation. All Mutables known by this factory are kept
  fully in RAM.
  
  Positive values of acceptableOverheadRatio will trade space for speed
  by selecting a faster but potentially less memory-efficient
  implementation. An acceptableOverheadRatio of COMPACT will make sure
  that the most memory-efficient implementation is selected whereas
  FASTEST will make sure that the fastest implementation is selected.
  */