_fq_reduce, _fq_sparse_reduce, _fq_dense_reduce

Reduce the polynomial (R, lenR), with arbitrary integer coefficients and $\mathit{lenR} \le 2d$ (any length for _fq_sparse_reduce), modulo $p$ and modulo the defining polynomial $f$ in place: the first $\min(\mathit{lenR}, d)$ coefficients receive the reduced value (and are not normalised). _fq_sparse_reduce eliminates the coefficients of degree at least $d$ from the top, using the nonzero terms of the monic version of $f$, and reduces modulo $p$ at the end; it costs about $(\mathit{lenR} - d) \cdot t$ multiplications for $f$ with $t$ nonzero terms. _fq_dense_reduce reduces modulo $p$ and then divides by $f$ with the precomputed inverse (Newton division, _fmpz_mod_poly_divrem_newton_n_preinv). _fq_reduce calls the first if $f$ has at most five nonzero terms and the second otherwise.