data block   // the bits to read
	off  uint32  // current offset into data
	bits uint32  // bits ready to be returned
	cnt  uint32  // number of valid bits in the bits field
}

// makeBitReader makes a bit reader starting at off.
func (r *Reader) makeBitReader(data block, off int) bitReader {
	return bitReader{
		r:    r,
		data: data,
		off:  uint32(off),
	}
}

// moreBits is called to read more bits.