Add `sum` methods for BigInts and BigFloats

As due to a pointer issue `mpfr_sum` isn't wrap-able, I've decided to write a slightly more efficient method for `BigFloat`s (and another one for `BigInt`s with the same structure) reusing the summation variable.
In my tests with arrays with 10^6 numbers, it seems to be about 5-10x faster than the generic version.