Return a number using the given quantity of signed bits.

def s_get(self, quant):
        """Return a number using the given quantity of signed bits."""
        if quant < 2:
            # special case, just return that unsigned value
            # quant can also be 0
            return self.u_get(quant)

        sign = self.u_get(1)
        raw_number = self.u_get(quant - 1)
        if sign == 0:
            # positive, simplest case
            number = raw_number
        else:
            # negative, complemento a 2
            complement = 2 ** (quant - 1) - 1
            number = -1 * ((raw_number ^ complement) + 1)
        return number