Potential changes for Universal Template Parameters

In the extractor documentation, it says "we would ideally like to write something like:

using list_t = csd::slist_head<ListItem, &ListItem::link>

Unfortunately we cannot do this directly."

This is because EntryEx must be either a type parameter or a non-type parameter, thus the need for all of our machinery for wrapping constexpr values in an invocable_constant and cinvoke_t helper aliases. If P1985 (Universal Template Parameters) moves forward from EWGI, we might be able to remove this complexity, see https://wg21.link/P1985