Used in a ReflectedModelField for xmap/xset fields.
A $interface is implemented in terms of a member FnTable which is a vtable of function pointers.
A PtrMapOrSetFns represents a pointer to the vtable either for interface IXMap or IXSet.
// Collects together a set of function pointers that allow clients of the reflection
// system to make use of a given xmap<Key,Value>
typedef IXMap::FnTable XMapFns;
// Collects together a set of function pointers that allow clients of the reflection
// system to make use of a given xset<Key>
typedef IXSet::FnTable XSetFns;
union PtrMapOrSetFns
{
const XMapFns* mapFns;
const XSetFns* setFns;
};