Speed up Cauchy products

We can improve on efficiency of Cauchy products by making sure that a Cauchy product of 3 series is defined as the Cauchy product of 2 series, where one of them is also a Cauchy product. For example, we can define A \star B \star C = A \star D, where terms of D = B \star C are cached and reused, speeding things up. Naive evaluation of A \star B \star C would require a quadratic number of products per order n, and this is what we do now. Using A \star D has a linear cost instead.