_fmpzi_poly_mulmid_classical_fft_small

Computes the coefficients [\mathtt{nlo}, \mathtt{nhi}) of the product of the Gaussian integer polynomials (poly1, len1) and (poly2, len2), with the coefficient arithmetic done pointwise on fft_small transforms: two transforms per coefficient (real and imaginary parts) and four pointwise operations per complex product, the shorter operand's transforms kept and the longer rolling as in _fmpz_poly_mulmid_classical_fft_small. Squaring shares transforms, pairs symmetric terms and computes diagonal squares with three multiplications. The lengths may be given in either order. Returns 1 on success and 0 when the transformed representation is unavailable or unprofitable in which case nothing is written and the caller should fall back to a different routine.