Initializes internal data representation of the BinArray to all-0.
        The internal data representation is simply tightly-packed bits of all
        words, starting from LSB, split into bytes and stored in a bytearray.
        The unused trailing padding bits in the last byte must always be set
        to 0.

def _init(self, width, len_):
        """Initializes internal data representation of the BinArray to all-0.
        The internal data representation is simply tightly-packed bits of all
        words, starting from LSB, split into bytes and stored in a bytearray.
        The unused trailing padding bits in the last byte must always be set
        to 0.
        """
        self._width = width
        self._len = len_
        bits = len_ * width
        self._data = bytearray(BinInt(bits).ceildiv(8))