Even after the great date/time consolidation, TypeCategory() was still

Even after the great date/time consolidation, TypeCategory() was still
a few bricks shy of a load concerning knowing all the date/time types.
This is real bad because it interferes with func_select_candidate()'s
willingness to disambiguate functions --- func_select_candidate() will
punt unless all the available choices have the same type category.
I think this whole mechanism needs redesigned, but in the meantime
this is a needed patch.