polys: Added wrappers of common functions to PolyElement

PolyElement now supports pdiv(), prem(), pquo(), pexquo(), half_gcdex(),
gcdex(), subresultants(), resultant(), discriminant(), decompose(),
shift(), sturm(), gff_list(), sqf_norm(), sqf_part(), sqf_list() and
factor_list(). All those methods are wrappers over dense representation.