Consider adding a GapObj subclass for polynomials

[embray]

GAP doesn't explicitly have a polynomial type (polynomials are "component objects" in the family of rational functions, specifically those with denominator 1).

It might be useful, for integration with Sage polynomials, if there were an explicit GapPolynomial type. It could also be callable, via calling the Value function.

Not exactly sure how useful this would be, but the current methods in Sage for converting Sage polynomials to libgap leave something to be desired. GapObjs on the other hand can be converted to Sage polynomials, at least in the case were IsUnivariateRationalFunction is true. Conversion of multivariate polynomials to Sage doesn't work, but mainly just because it hasn't been implemented explicitly.