polys: Fixed construct_domain() to work under Python 2.5
solvers: Fixed test failure in test_solvers.py (test_exclude)
polys: Better guard against CoercionFailed in PolyElement
polys: Use specialized PolyElement/FracElement in PolyRing/FracField
polys: Use dtype in isinstance() in PolyElement/FracElement
polys: Renamed monomials() to itermonomials() to fix import issues