Initial support for BigIntegers

Not sure what we should do about division: Currently it uses `floordiv` in rpython, which probably isn't what we want.

We could change Ratio to always store BigIntegers as numerator and denominator, like Clojure does, but I'm not sure if we should do that. Otherwise, we could add a BigRatio, but it would make the code more complex. I think I tend towards Ratios with BigInteger fields.

@halgari What do you think?