fmpz_mpoly_hlift

Lifts the $r \ge 2$ factors $f_i$ of $A \equiv \prod_i f_i \bmod (x_m - \alpha_{m-1})$, in a lex context with single-word exponents, to factors in the variables $x_0, \ldots, x_m$. The leading coefficients of the $f_i$ in $x_0$ must already be the correct ones (polynomials in all of $x_1, \ldots, x_m$), and degs are the degrees of $A$. The lifting runs up to degree degs[m] in $x_m - \alpha_{m-1}$; it uses the error update of the quartic method, specialised for $r = 2$, when $r < 20$, and recomputes the product of the factors (quintic method) otherwise. Returns 0 if a partial fraction problem has no solution or the degrees exceed degs[m], in which case there is no factorisation of $A$ with these images and leading coefficients. Only the quintic version checks at the end that $A = \prod_i f_i$; the callers verify the factors by division in any case. See The lifting step.