_fft_small_mpn_mul_complex, _fft_small_mpn_sqr_complex

Only the nonzero parts are transformed, so a real or imaginary operand costs one transform for both output parts. Each output part is a sum of at most two products, formed with its signs in transform space and exported signed; an output part that is a single product with $\mathit{lo} = 0$ is exported unsigned with its known sign, and if all are, the plan is unsigned and packs more bits per chunk. Storage for five transforms is needed for a product and three for a square. A full product ($\mathit{lo} = 0$, not a square) of an operand with a single nonzero part by a complex one consists of two products with a common factor and is passed to _fft_small_mpn_mul_shared when the outputs do not overlap the operands in a way that function forbids.