The old polynomial ring currently prints in the same way as the
new one and its string representation only creates the new ring.

>>> A = ZZ.old_poly_ring(x)
>>> sstr(A)
'ZZ[x]'
>>> srepr(A)
'ZZ[x]'
>>> type(ZZ[x])
<class 'sympy.polys.domains.polynomialring.PolynomialRing'>
>>> type(A)
<class 'sympy.polys.domains.old_polynomialring.GlobalPolynomialRing'>

This commit defines a representation string that can be used to
construct the old polynomial ring.

>>> srepr(A)
"GlobalPolynomialRing(ZZ, Symbol('x'))"

With fraction fields, there is another issue. The printed string
of type QQ(x) cannot be used at all to create a SymPy object.
Therefore a new representation is defined.

>>> K = QQ.frac_field(x)
>>> sstr(K)
'QQ(x)'
>>> srepr(K)
"FractionField(FracField((Symbol('x'),), QQ, lex))"