Removed obsolete buildnuget.bat file

- "build.bat Release" can now be used to build the nuget packages
Fixed stack overflow in recently refactored FdbTuple.Append<...>(...) extension methods

- Need to cast result of FdbTuple.Create<...>(...) to IFdbTuple first, or else it will call the Append<T> override and not the Append(IFdbTuple) override