acb_mzeta_polylog

Sets res to the multiple polylogarithm

(the convention of PARI/GP's polylogmult). The value is computed from Goncharov's iterated integrals $G(a_1, \ldots, a_L; 1)$ with letters $a_i \in \{0, 1/y_1, \ldots, 1/y_r\}$, $y_i = x_1 \cdots x_i$, as power series at 0 by binary splitting, either directly (when all nonzero letters have absolute value at least 2) or after splitting the path at $\tfrac12$ (when the nonzero letters $a_i$ and $1 - a_i$ have absolute value at least $\tfrac34$); see Multiple polylogarithms. Other arguments, for example $x$ outside the unit disk or letters close to $\tfrac12$, are not supported yet, and the result is then indeterminate, as it is for divergent sums ($s_1 = 1$, $x_1 = 1$).