Converts the word to a BinInt, treating it as a signed number.

def to_sint(self):
        """Converts the word to a BinInt, treating it as a signed number."""
        if self._width == 0:
            return BinInt(0)
        sbit = 1 << (self._width - 1)
        return BinInt((self._val - sbit) ^ -sbit)