Remove $Math::GMPz::RETYPE and all references to it. (The option to turn off the cross-class operator overloading no longer exists.)
Allow strings and NVs with the overloaded pow operators (**' and '**=').
Simplify "overloading" documentation.
Allow Math::GMPz to overload cmp ops between Math::GMPz and Math::MPFR objects. Previously such comparisons could be handled only by the Math::MPFR overloading.