MonomialEval started Horner evaluation from poly[len-2] instead of the
highest-degree coefficient poly[len-1], so it silently ignored the top
coefficient (1 + 2x at x=3 gave 1 instead of 7) and indexed poly[-1] on a
constant polynomial, panicking. Start from poly[n] and iterate from n-1,
matching the documented y = sum x^i * poly[i].
Closes#579
