Overflow when computing coefficient * weight

[mdbenito]

math.comb(n, k) * (1/2**(n-1)) can raise an OverflowError pretty quick inside the coefficient_fun() built in EvaluationStrategy, but math.comb(n, k) / 2**(n-1) won't. We probably need to return a callable in the coefficient, or hardcode the division instead of the multiplication 🙈