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.