polys: Optimized check if PolyElement is a generator
polys: Simplified and optimized PolyElement.{__add__,__sub__}
polys: Extracted eqs_to_matrix() from solve_lin_sys()
polys: Added new solve_lin_sys() benchmark (10x8 GCD extensive)
polys: Don't reimplement global namespace modification hack
polys: Distinguish symbols from generators in domains
polys: Allow to pre-process domain in PolyRing()