Computes the composition of polynomials P1(t) and P2(t), that is P1(P2(t))
It uses interpolation to determine the evaluation at points x_i and evaluates
P1(P2(x[i])). The interpolation theorem ensures that we can reconstruct the polynomial
uniquely by interpolation over a suitable number of points
This is an inefficient version, for something more efficient, use FFT for evaluation,
provided the field satisfies the necessary traits