_fmpz_poly_cyclotomic, fmpz_poly_cyclotomic

The underscore version requires n \ge 3 squarefree, the array factors of its num_factors prime factors (with 2 first if n is even) and \varphi(n) as phi, and writes only the low \lfloor (\varphi(n) + 1)/2 \rfloor coefficients; the others follow by symmetry, since \Phi_n is palindromic for n \ne 1. It uses the sparse power series algorithm (Algorithm 4) of [ArnoldMonagan2011], based on \Phi_n(x) = \prod_{d \mid n} (x^d - 1)^{\mu(n/d)}, in which multiplications and divisions by x^d - 1 cost O(\varphi(n)) additions, for O(2^k \varphi(n)) operations in total for k prime factors. Machine integers are used when no overflow is possible: by Table 6 of [ArnoldMonagan2011], all coefficients of \Phi_n have at most 27 bits for n < 10163195 and at most 60 bits for n < 169828113, and they are 0, \pm 1 when n has two prime factors. The cases n = p and n = 2m with m odd (\Phi_n(x) = \Phi_m(-x)) are handled directly.