gr_poly_factor_equal_deg, _gr_poly_factor_equal_deg_with_frob

This is the Cantor-Zassenhaus algorithm [CanZas1981] in the trace form of von zur Gathen and Shoup [GatSho1992]: for random $a \in \mathbb{F}_q[x]/(f)$, the trace $b = a + a^q + \cdots + a^{q^{d-1}}$ (computed by modular composition with the Frobenius $x^q$, by exponentiation, or by a doubling scheme, depending on $q$ and $d$) lies in the Berlekamp subalgebra, and $\gcd(f, b^{(q-1)/2} - 1)$ for odd $q$, or $\gcd(f, b + b^2 + \cdots + b^{2^{k-1}})$ for $q = 2^k$, splits $f$ with probability about $1/2$. The splitting is repeated on the factors. For $d = 1$ (root finding) $a$ is a random linear polynomial, which makes the exponentiation cheaper. When several pieces remain and threads are available over a thread-safe ring, the pieces are split in parallel, each with its own random state. The underscore version assumes $f$ monic, takes $x^q \bmod f$ as frob (unused for $d = 1$) and a random state, and appends to fac.