Change the C implementation of `num_smallest` primitive to return `DBL_MIN` instead of `DBL_EPSILON`, correcting the value from the machine epsilon (difference between 1.0 and next representable value) to the smallest positive representable double. Update the documentation in `num.markdown` to describe `Num.smallest` as "the smallest positive representable numeric value" and `Num.largest` as "the largest representable numeric value". Add test files `largest.wren` and `smallest.wren` verifying the expected output values.
