_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.