Itoh–Tsujii multiplicative inversion in GF(2^n): computes a^{-1} = a^(2^n - 2) = (a^(2^(n-1) - 1))^2 using a generic binary addition chain on the exponent e = n - 1, driving the supplied IBinPolyMul's Multiply / Square / SquareN (so it is backend-independent — one implementation serves every ISA).