Work around BigInteger not having expt

BigInteger doesn't actually implement exponentiation. For now, I added something that redirects through regular numbers, but this doesn't work for `expt 2 100` and other large exponents. It returns a `NaN` value, packaged inside of a number.

Really, someone should implement the iterated multiplication definition of exponentiation for BigNums. I'm currently preoccupied with final projects though.