Number of small fixes, first (not-completely-working) version of modexp

created function BigInt::div that is used by operator %= and /=, and can be used to return both the remainder and quotient from a division

Created functions BigInt::pow(exp, mod), BigInt::modexp_montgomery(base, exp, mod), and BigInt::modexp_sliding_window(base, exp, mod). the sliding window method is not yet implemented