Function k_lopsided_mul

b has at least twice the digits of a, and a is big enough that Karatsuba * would pay off *if* the inputs had balanced sizes. View b as a sequence * of slices, each with a->ob_size digits, and multiply the slices by a, * one at a time. This gives k_mul balanced inputs to work with, and is * also cache-friendly (we compute one double-width slice of the result * at a time, then move on, never