rsolve_poly/hyper fixes
rsolve_poly/hyper fixes

1) cancel common factors early in rsolve_poly,
   e.g. for rsolve_poly([-n - 1, n + 1], n*(n + 1)**2, n)

2) in rsolve_hyper we skip initial zero/infinite terms of the
   product of ratio's, if one has zeros or poles as function of n.
   For example: rsolve_hyper([-n, 1], 0, n).

skirpichev/omg,diofant/diofant